Brain.save_movie() II

Alternative to https://github.com/nipy/PySurfer/pull/79 co-opting `Brain.make_montage()`

If you're feeling motivated:

http://stackoverflow.com/questions/2017843/fetch-frame-count-with-ffmpeg

Can be left for later if you want, but it would be a nice check. If you go with the first one-liner command solution, note the sub-reply about using `-an` to ignore audio.