missing header file in some tutorial code (Bug #2800)

The tutorial code about features2d in recent opencv release won't work as expected, because they all used classes about surf and sift, which are no longer declared in the "opencv2/features2d/features2d.hpp" file. Instead, they are declared in "opencv2/nonfree/nonfree.hpp". Therefore, each of them should contain a #include line of this file to work properly.
Further more, the "opencv_nonfree242.lib" shoud be added in the linker input.

In tutorial 6.1 "Feature Description", line 51:
BruteForceMatcher< L2<float> > matcher;
BruteForceMatcher class has been renamed as BFMatcher, and should be constructed like:
BFMatcher matcher(NORM_L2);