Brain.save_movie() II

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

@Eric89GXL I updated the signature, the example works. As for your remark about the complex signature, maybe we can have the first part of the function signature be what normal users would use (in terms of timing `time_dilation=4., tmin=None, tmax=None` should be all that's needed, for montage we'd have to think about it) and the latter part include more options for power users? 

What do you think about changing the default interpolation to "quadratic"? The example looks much smoother that way.