Added the defaultNorm() method to the DescriptorExtractor class. This method returns the default norm type for each descriptor type. The tests and C/C++ samples were updated to get the norm type directly from the DescriptorExtractor inherited classes.

This was reported in feature report #2182 (http://code.opencv.org/issues/2182). It will make it possible to get the norm type usually applied matching method for each descriptor, instead of passing it manually.

HAVE_opencv_ocl implies HAVE_OPENCL, so checking for both is not
necessary.

2. Let Farneback to be the default optical flow method.
3. Fix a timing method bug for ocl path.
4. Remove useless finish operation in farneback
5. Refactor buffer usage.

create `d_fgimg` matrix before cleaning.

Allow to build samples via OpenCV binaries from distribution package
(find_package with OpenCV_DIR).

Conflicts:
	modules/core/include/opencv2/core/cuda.hpp
	modules/cudacodec/src/thread.cpp
	modules/cudacodec/src/thread.hpp
	modules/superres/perf/perf_superres.cpp
	modules/superres/src/btv_l1_cuda.cpp
	modules/superres/src/optical_flow.cpp
	modules/videostab/src/global_motion.cpp
	modules/videostab/src/inpainting.cpp
	samples/cpp/stitching_detailed.cpp
	samples/cpp/videostab.cpp
	samples/gpu/stereo_multi.cpp

* use different approaches -> threads and streams
* clean up code

2. Modified according to the feedback of the community.

converted it to abstract interface

removed set/get methods from Algorithm (implement owns)
removed GHT_* enumeration