links BoW and Feature Descriptors

http://public.cranfield.ac.uk/c5354/teaching/dip/opencv/lecture_demos/c++/surf_feature_matching.cpp
http://www.morethantechnical.com/2011/08/25/a-simple-object-classifier-with-bag-of-words-using-opencv-2-3-w-code/

https://code.ros.org/trac/opencv/browser/trunk/opencv/samples/c/detectors_sample.cpp?rev=3052

http://opencv.itseez.com/doc/user_guide/ug_features2d.html
http://app-solut.com/2011/07/the-bag-of-words-model-in-opencv-2-2/