Added feature #2753 : difference between VS2010 and VS2012

Added suggested windows environment variable for $OPEN_CV$, which is basically the only difference for using VS2012 with the openCV library.

http://code.opencv.org/issues/2753

Also changed an argument, related to a fix that was already merged : https://github.com/Itseez/opencv/pull/1136
Due to changing the guide there, mentioning that include folder is NOT in the $OPEN_CV$ folder, i deleted this from the comment in this guide also.