all video processing samples use camera as default source (and fallback to synth in case of capture error)

Added the "Camera calibration With OpenCV" tutorial. Extended with a few global links the conf.py file. Corrected an error in the linux_install.rst and one in the cascade_classifier.rst file. 

updated retina modules : minor misakes correction & changed pixel format to float instead of double to keep some memory, precision is almost sufficient, check for residual mistakes

updated retina modules : minor misakes correction & changed pixel format to float instead of double to keep some memory, precision is almost sufficient, check for residual mistakes

Added the "How to write a tutorial for OpenCV?" tutorial. Made some corrections to the feature2D TOC tree. Small visual improvements for the video-write.rst file. Deleted a definition file that served no purpose.

fixed crashes in parallel HOG detector; avoid use of new POPCNT instruction in ORB (to make it compatible with older CPUs).