Optical flow - testing app and additional interfaces

This is a part of my GSoC contribution

Includes a testing app - computing stats about algorithms' performance, strongly inspired by Middlebury benchmark - placed in samples, in optflow module. 
Additionally:
+ helper functions for reading and writing .flo files
+ object oriented interfaces for optflow algorithms - like the ones in superres module, but in a more logical place.

Testing app is using object-oriented interfaces from opencv_contrib/optflow and opencv/video (not opencv/superres)

-- 
Note:
Algorithm::info()-related code disabled-after moving to opencv_contrib caused compilation errors