Added the How to create videos with OpenCV Tutorial. Made some corrections to the feature2d, to get rid of some documentation build errors. 

minor cleaning

* Fixed display of struct and class definitions (except classes with base class specified).
* Fixed formatting and referencing errors in several documentation files.

Added tutorial for features2d using homography to find a planar object (Based on the well known find_obj.cpp

added cv.py micro-module for backward compatibility with existing Python scripts using prev-gen Python API.

some old cv api use cleaning

fixed kmeans center initialization in the case of huge matrices (ticket #1291); fixed warinings in libtiff.

added support for Ximea cameras (patch by Marian Zajko), fixed GCC warnings in MOG2 background subtractor.