moved nonfree and a part of features2d to opencv_contrib/xfeatures2d
fixed doc builder complains and the test failures
removed the stuff that's now in xfeatures2d; temporarily added dummy definition of SIFT to make doc builder pass (will remove it later)
removed the stuff that's now in xfeatures2d; temporarily added dummy definition of SIFT to make doc builder pass (will remove it later)
fixed more doc builder warnings
removed some more trailing whitespaces
Merge pull request #3078 from vpisarev:refactor_features2d_take3