The y4m extension used is the same as the one used in ffmpeg/x264.
The patch is adapted from the highbitdepth branch.

Also adds unit tests for y4m header parsing and md5 check
of the raw frame data, as well as y4m writing.

[build fix for Mac/VS by not using tuples with strings]

Change-Id: I40897ee37d289e4b6cea6fedc67047d692b8cb46