Brain.save_movie() II

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

Haha yeah my choice of 25 probably comes from PAL. it's more than enough for perceiving fluid motion. If you insist I'll make it 30. What you're referring to with 60 though is 60 half frames, which we don't want.