Create maintenance branch for 0.13
@jreback and I were discussing and we think it makes sense to freeze features for 0.13 and have master focus on 0.14-dev. Just wanted to throw it out there to make sure everyone is okay with this.  After this point, we're only going to allow doc changes and, until 0.13 is released, we'll backport bug fixes as well.

I think it's better to keep master as bleeding edge so it's simpler for new contributors.  If no one objects, wherever master is 24 hours-ish from this issue I'll mark current master as 0.14-dev and checkout a separate `rc/0.13` branch.
