Brain.save_movie() II

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

This could be simplified, combining four parameters into one:
```
times : slice | array-like
    If slice, the set of indices to use, e.g. ``slice(None, 10, 2)``. Can also be
    an array of indices.
```