Fixed color code in cvtColor in "Load, Modify, and Save an Image" tutorial (bug #2739)
Added nonfree header in "Feature description" tutorial code sample (bug #2527)
Merge pull request #356 from Daniil-Osokin:bugfix_imread_tutorial
Merge pull request #364 from Daniil-Osokin:bugfix_doc_features2d_tutorial