Brain.save_movie() II

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

are you suggesting interpolating the data between samples? I don't think that's possible currently right? I agree it would be the nicest solution eventually. Maybe in a separate PR though, since I don't think it's trivial(?) 

If we have interpolation, then `step=0.5` is ambiguous again because it could mean step forward 0.5 seconds, or step forward half a frame (i.e. interpolate every other frame)... It would be more explicit to add a `tstep` argument to specify the step in time vs frames (once interpolation is possible)...

I like having `time_dilation` as the primary argument, but I guess it also does not really make sense until we can interpolate...

To keep it simple I would suggest to drop time_idx (unless there is a good use case for it) and keep `step` in samples, leaving the interpolation and better timing control for later?

Regarding the signature, I tend to prefer several simple arguments to fewer but more complex argument, i.e. I would prefer 
```
brain.save_movie(fname, tstart=0.1)
```
to
```
brain.save_movie(fname, times=slice(0.1, None))
```
but I can implement the latter if that is generally preferred.