Brain.save_movie() II

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

maybe default time_dilation to 4 (because evoked data is usually less than a second)? Also I think that will usually lead to very high frame rates (200 Hz data, time dilation 1 would be 200 fps...). I am not sure how well different systems play those movies, should we impose a default framerate? Or do people want all samples in the movie by default so that they can scroll through manually?