Brain.save_movie() II

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

@Eric89GXL I've simplified the function signature quite a bit, however realized a new complication. The montage code does not take into account that there can be two hemispheres. So currently you can only make multi hemisphere movies with montage='current'. I've been using that, and it always seemed to work as expected, but the function involved does come with that warning:
```
        Due to limitations in TraitsUI, if multiple views or hemi='split'
        is used, there is no guarantee painting of the windows will
        complete before control is returned to the command line. Thus
        we strongly recommend using only one figure window (which uses
        a Mayavi figure to plot instead of TraitsUI) if you intend to
        script plotting commands.
```
If this is still current, I feel like for a clean solution the montage code should be modified such that you could specify a montage like 
```
[['lh lat', 'rh lat'].
 ['lh med', 'rh med']]
```
WDYT?