Brain.save_movie() II

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

You could use linear interpolation in such cases, but that *could* lead to undesirable effects in some circumstances. We could allow `step` to be in integer or a float, and say that one implies time indices and the other implies time steps. That would require users to be very careful about their arguments, but also be the most flexible solution. If it's a float, we do interpolation if necessary. We can just say in the docstring that `1` is different from `1.` in this regard. The advantage to this approach is that it makes it very easy to make a smooth movie, e.g. by doing `times=slice(-0.2, 1.0, 1./60.), framerate=60.`, or to create a more jumpy but un-interpolated movie as `times=slice(-0.2, 1.0), framerate=fs`. WDYT?

Also, at some point we should say what the effective output rate is, e.g. if I do `times=slice(-0.1, 1.0, 1./600.) framerate=60` then it should tell me that it's going to show 10x slower than realtime.

Also, maybe the user should supply a `time_dilation` argument instead of a `framerate` argument. I think in most cases, users will want to play their movie at some speed relative to realtime, as opposed to hitting some specific output frame rate. In cases where they do want a specific frame rate, *then* we make them do the simple arithmetic to work it out.