Brain.save_movie() II

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

I think it depends on the future of the montage code. I suspect that with a new visualization tool the layout of the display figure will become more flexible, and the whole montage code will be deprecated. If that's the case we might as well leave this for now and implement a clean new function once it's exactly clear how it will work.