Chunk for rate-limiting

On a repository with a large number of pulls (somewhere around 1.5k), we run out of the 5000 API calls we get in an hour, at which point the script dies.

Although it's certainly going to take a lot longer, it'd be nice if we slept until we could make requests successfully again.

I've got an incredibly inelegant hack I'm testing, but I'd like to see how sigmavirus24/github3.py#129 shapes up.
