fixed many bugs related to Mat::getUMat(), asynchronous kernel execution etc. Also, played a bit with ocl::cvtColor vs cv::cvtColor performance

Conflicts:
	modules/ocl/src/arithm.cpp

Conflicts:
	.gitignore
	CMakeLists.txt
	doc/CMakeLists.txt
	modules/calib3d/src/stereosgbm.cpp
	modules/core/include/opencv2/core/mat.hpp
	modules/highgui/src/cap_openni.cpp
	modules/ml/include/opencv2/ml/ml.hpp
	modules/objdetect/src/hog.cpp
	modules/ocl/perf/perf_color.cpp
	modules/ocl/src/arithm.cpp
	modules/ocl/src/filtering.cpp
	modules/ocl/src/imgproc.cpp
	modules/ocl/src/optical_flow_farneback.cpp
	platforms/scripts/camera_build.conf
	platforms/scripts/cmake_android_all_cameras.py
	samples/cpp/Qt_sample/main.cpp
	samples/cpp/tutorial_code/introduction/windows_visual_studio_Opencv/Test.cpp