Brain.save_movie() II

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

I don't like parameter explosion, but we might need to use `tmin=None, tmax=None, tstep=None, framerate=1/60., time_dilation=4.`, where only two of `[tstep, framerate, time_dilation]` can be non-`None`. I also prefer `tmin/tmax` to match what `mne-python` does, is there a reason to use `tstart` / `tstop` here?