Provide option to not show buttons in JSAnimations (slider only).

For some animations, "play" just doesn't make sense.  Perhaps the different frames aren't a time sequence at all, or there are only a few frames, etc.  This allows one to just put a slider below the plot with no other buttons.

Leaves the default behavior unchanged.

Demo here, showing old and new behavior: http://nbviewer.ipython.org/gist/ketch/9522130

@jakevdp if this seems like it would be useful to others, I can submit a PR to your repo (or feel free to just grab a patch from here).