This patch adds ocl kernels to accelerate Dense Inverse Search
based optical flow algorithm, it acclerates 3 parts in the algorithm,
including 1) Structure tensor elements compute, 2) Patch inverse search,
3) Densification compute.

Perf and accuracy test are also added. The perf test shows it is 30%
faster than the current implementation.
Signed-off-by: Li Peng <peng.li@intel.com>

* added BackgroundSubtractorCNT

* doc fixes

* fixed typo

Currently if you are building OpenCV with contrib under ios, not as framework, this can leads to errors in config.
I suppose it would be better if xobjdetect module app will be optional with the BUILD_opencv_apps just like others tools in OpenCV

Wrap ICP for Python