facerecognizer interface + white + whitespaces remove

ReST directive was used. Also fixed some other ReST directives that were
not correct and removed some warnings during buildbot checks.

- Used lunix style slashes - works on most systems
- Removed all trailing whitespaces

Also, removed the one from modules/python/src2/cv.py and cleared its
executable bit, since it's not a script.

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

Added a simple CMakeLists.txt to the facerec_samples, which are not included in the samples folder. Makes building these samples a lot easier.

FaceRecognizer supports updating a model now. Documentation has been updated to reflect latest changes.

Updated the documentation to (1) reflect the latest changes and (2) answer some questions I were asked.

Fixed facerec_eigenfaces.cpp demo sample: it contained hardcoded values, which made wrong element access possible. Fixed unsigned integer/signed integer warnings in facerec.cpp.

Grammar and Formatting fixes. Added link to the Yale Facedatabase A. Added download directive links for linking to the sample source code samples.