Brain.save_movie() II

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

Well my reasoning was that `tstart` and `tstop` in seconds would be most user friendly (users know what time the movie should start and stop, but probably would have to calculate the index that that corresponds to). Would you consider step in seconds intuitive? (i.e. between two frames make a step of 0.01 seconds). The only problem I see is that integers are always valid, but say your sampling rate is 250 Hz, what should we do if the user says step=`0.01`?

I added time_idx because it's an argument for the image sequence command, and because there are cases that start/tstop don't cover, but I don't think it's very user friendly. I would be happy to drop time_idx though, don't think I would ever use it.