Clarified floodfill documentation

Clarified what value floodfill sets pixels in the mask to when FOODFILL_MASK_ONLY is set.
Merge pull request #828 from tim36272:patch-1
gpucodec module for video decoding/encoding
gpuarithm module for arithmetics operations on matrices
gpufilters module for image filtering
CV_GPU_TEST_MAIN macros
gpunvidia module for NCV & NPP API
removed columnSum function (it is a duplicate for reduce)
added private header to gpunvidia module
replaced NCV_EXPORTS with CV_EXPORTS
made objdetect dependecy for gpunvidia optional
moved rectStdDev to gpuarithm
moved copyMakeBorder to gpuarithm module
moved integral to gpuarithm module
moved mulSpectrums, dft and convolve to gpuarithm
gpuimgproc module for image processing
gpufeatures2d module 2d for feature detection and matching
gpuvideo module for video processing
gpucalib3d module for camera calibration and stereo correspondence
gpuobjdetect module for object detection