Brain.save_movie() II

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

Let's go with `tmin=None, tmax=None, time_dilation=1., interpolation='linear'`. For linear interpolation, you can just wrap to `scipy.interpolate.interp1d` along the time axis (`axis=1`). If it doesn't make sense to you, just implement the movie parts and I'll take over to make a PR into your branch for the interpolation.