Brain.save_movie() II

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

> Can we more or less just make this a wrapper to save_image

it was a wrapper around `save_single_image`, I made it a wrapper that decides whether to wrap itself around `save_single_image`, `save_image` or `save_montage`. Maybe a separate wrapper for save_image would be preferable? Too bad that the name save_image_sequence is taken by this one... 

> This one at least doesn't need interpolation since you pass it a lost of indices, right

Yes... that is because that is how the existing function works... I think ideally all that would be handled by the function that makes the frames, and the movie function would only make the frames and combine them into a movie, no?