added test to save image with webp with alpha channel
switched to Input/Output Array in gpuwarping functions
refactored gpu::ImagePyramid (converted it to abstract interface)
updated documentation
added support image with alpha channel
updated webp to version 0.3.1
Merge pull request #1068 from AoD314:webp2
Merge pull request #979 from jet47:gpuwarping-refactoring