Brain.save_movie() II

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

...although I guess you could still use floats with `slice`, so I think the above might still work. Now that I think about it a bit more, why make it so tstart and tstop are float but step is an integer? This is counter-intuitive to me -- it's probably better to have all three be integers (indices), or all three be floats (times).