improved fix for java wrapper generator (gen_java.py) to avoid generation of java methods with duplicate signatures(v3)

* fix for feature2d java wrappers as described in this post: http://answers.opencv.org/question/101675/surfsift-java-wrapper-for-opencv-3xosx-1011/

* fix for feature2d java wrappers as described in this post: http://answers.opencv.org/question/101675/surfsift-java-wrapper-for-opencv-3xosx-1011/

* rollback of one change as requested (similar change already merged)

Fixed small typo for python 3 default version.
Signed-off-by: Noel Eck <noel.eck@intel.com>

* normHamming done with AVX 2 intrinsics.

* fixed compiling AVX2-optimized norm hamming with MSVC

make sure that the empty mat is copied to UMat properly - i.e. UMat becomes empty. Before the patch such copy operation crashed

1) fix include directories
2) don't use PCH for source file with custom COMPILE_FLAGS