Brain.save_movie() II

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

Nah, NTSC is too old school -- the number 60 popped into my head because 720p, Blu Ray 1080p, and most modern computer displays all run at that actual frame rate. But space saving is a compelling argument, so let's do `30.` for the default. I'll just use `60.` when I make the movies :)