reg module: Add sample and README file

README.md file with a short description of the module is added. An
example on how to use the module can be found now in reg/samples/.

modules/reg/README.md

+# OpenCV pixel-intensity based registration module
+
+Author and maintainer: Alfonso Sanchez-Beato
+                       alfonsosanchezbeato\_\_\_\_gmail.com
+
+These classes implement a module for OpenCV for parametric image registration.
+The implemented method is direct alignment, that is, it uses directly the pixel
+values for calculating the registration between a pair of images, as opposed to
+feature-based registration.  The implementation follows essentially the
+corresponding part of the paper "Image Alignment and Stitching: A Tutorial",
+from Richard Szeliski.
+
+Feature based methods have some advantages over pixel based methods when we are
+trying to register pictures that have been shoot under different lighting
+conditions or exposition times, or when the images overlap only partially. On
+the other hand, the main advantage of pixel-based methods when compared to
+feature based methods is their better precision for some pictures (those shoot
+under similar lighting conditions and that have a significative overlap), due to
+the fact that we are using all the information available in the image, which
+allows us to achieve subpixel accuracy. This is particularly important for
+certain applications like multi-frame denoising or super-resolution.
+
+In fact, pixel and feature registration methods can complement each other: an
+application could first obtain a coarse registration using features and then
+refine the registration using a pixel based method on the overlapping area of
+the images. The code developed allows this use case.
+
+The module implements classes derived from the abstract classes cv::reg::Map or
+cv::reg::Mapper.  The former models a coordinate transformation between two
+reference frames, while the later encapsulates a way of invoking a method that
+calculates a Map between two images.  Although the objective has been to
+implement pixel based methods, the module could be extended to support other
+methods that can calculate transformations between images (feature methods,
+optical flow, etc.).
+
+Each class derived from Map implements a motion model, as follows:
+
+* MapShift: Models a simple translation
+
+* MapAffine: Models an affine transformation
+
+* MapProject: Models a projective transformation
+MapProject can also be used to model affine motion or translations, but some
+operations on it are more costly, and that is the reason for defining the other
+two classes.
+
+The classes derived from Mapper are
+
+* MapperGradShift: Gradient based alignment for calculating translations. It
+produces a MapShift (two parameters that correspond to the shift vector).
+
+* MapperGradEuclid: Gradient based alignment for euclidean motions, that is,
+rotations and translations. It calculates three parameters (angle and shift
+vector), although the result is stored in a MapAffine object for convenience.
+
+* MapperGradSimilar: Gradient based alignment for calculating similarities,
+which adds scaling to the euclidean motion. It calculates four parameters (two
+for the anti-symmetric matrix and two for the shift vector), although the result
+is stored in a MapAffine object for convenience.
+
+* MapperGradAffine: Gradient based alignment for an affine motion model. The
+number of parameters is six and the result is stored in a MapAffine object. 
+
+* MapperGradProj: Gradient based alignment for calculating projective
+transformations. The number of parameters is eight and the result is stored in a
+MapProject object.
+
+* MapperPyramid: It implements hyerarchical motion estimation using a Gaussian
+pyramid. Its constructor accepts as argument any other object that implements
+the Mapper interface, and it is that mapper the one called by MapperPyramid for
+each scale of the pyramid.
+
+If the motion between the images is not very small, the normal way of using
+these classes is to create a MapperGrad\* object and use it as input to create a
+MapperPyramid, which in turn is called to perform the calculation. However, if
+the motion between the images is small enough, we can use directly the
+MapperGrad\* classes. Another possibility is to use first a feature based method
+to perform a coarse registration and then do a refinement through MapperPyramid
+or directly a MapperGrad\* object. The "calculate" method of the mappers accepts
+an initial estimation of the motion as input.
+
+When deciding which MapperGrad to use we must take into account that mappers
+with more parameters can handle more complex motions, but involve more
+calculations and are therefore slower. Also, if we are confident on the motion
+model that is followed by the sequence, increasing the number of parameters
+beyond what we need will decrease the accuracy: it is better to use the least
+number of degrees of freedom that we can.
+
+In the file map_test.cpp some examples on how to use this module can be seen.
+There is a test function for each MapperGrad\*. A motion is simulated on an input
+image and then we register the moved image using a MapperPyramid created with
+the right MapperGrad\*. The difference images of the pictures before and after
+registering are displayed, and the ground truth parameters and the calculated
+ones are printed. Additionally, two images from a real video are registered
+using first SURF features and then MapperGradProj+MapperPyramid. The difference
+between the images and the difference of the registered images using the two
+methods are displayed. It can be seen in the differences shown that using a
+pixel based difference we can achieve more accuracy.
+

modules/reg/samples/CMakeLists.txt

+cmake_minimum_required(VERSION 2.8)
+project(map_test)
+find_package(OpenCV REQUIRED)
+
+set(SOURCES map_test.cpp)
+
+include_directories(${OpenCV_INCLUDE_DIRS})
+add_executable(map_test ${SOURCES} ${HEADERS})
+target_link_libraries(map_test ${OpenCV_LIBS})