Fix findContours crash for very large images (#7451)

* Cast step to size_t in order to avoid integer overflow when processing very large images

* Change assert to CV_Assert

* 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