Added a test that documents that negative numbers are not clipped by cv::saturate_cast
fix performance issue of gpu reduction
Build info output for all android apps added.

OpenCV build information output to logcat added to OpenCV initialization code;
Documenation for cv::getBuildInformation() added.
Merge pull request #468 from kirill-kornyakov:test-for-saturate-cast
Merge pull request #478 from jet47:gpu-reduction-fix
Merge pull request #477 from asmorkalov:android_version_info
Fix warnings from ffmpeg wrapper
Merge pull request #480 from taka-no-me:ffmpeg_warnings