In Python 3, version 3.3+, PyUnicode_AsUTF8() provides similar
functionality to Python 2's PyString_AsString().

In older versions of Python 3, there is no public function to provide
the same functionality.  However, the "internal" _PyUnicode_AsString()
does provide that functionality, so use it to replace
PyString_AsString().

With this patch, cv2 should compile for Python 3.[0-2].

Conflicts:
	modules/java/generator/src/cpp/VideoCapture.cpp

In master, it's already set to NEW, since we declare the minimal CMake
version as 2.8.7, which is newer than the policy.

Conflicts:
	cmake/OpenCVDetectCUDA.cmake
	modules/core/include/opencv2/core/version.hpp
	modules/cudacodec/src/ffmpeg_video_source.cpp
	modules/gpu/src/video_writer.cpp
	modules/highgui/test/test_ffmpeg.cpp
	modules/highgui/test/test_video_io.cpp
	modules/highgui/test/test_video_pos.cpp
	modules/ocl/include/opencv2/ocl/ocl.hpp
	modules/ocl/include/opencv2/ocl/private/util.hpp
	modules/ocl/src/arithm.cpp
	modules/ocl/src/blend.cpp
	modules/ocl/src/canny.cpp
	modules/ocl/src/cl_operations.cpp
	modules/ocl/src/filtering.cpp
	modules/ocl/src/haar.cpp
	modules/ocl/src/imgproc.cpp
	modules/ocl/src/kmeans.cpp
	modules/ocl/src/moments.cpp
	modules/ocl/src/safe_call.hpp
	modules/ocl/src/split_merge.cpp
	modules/ocl/test/test_moments.cpp
	samples/ocl/squares.cpp

removed test for double since not all platforms support double, thus the sanity check cannot pass on all the platforms.