Added the tutorial on background subtraction, in the video section
Added a small modifications in the code formatting of background subtraction tutorial
added a brief description for the learning rate parameter in the apply method
Merge branch 'master' of https://github.com/Itseez/opencv into add_bs_tutorial

Conflicts:
	modules/stitching/src/motion_estimators.cpp
trying to solve with space errors
fixing other white spaces
fixing trailing whitespaces
fixing trailing whitespaces
fixing trailing whitespaces
Merge pull request #1315 from dbloisi:add_bs_tutorial