Upgrade github3.py dependency.

While there are a few bugfixes and other nice things, this was primarily
motivated by the script not working when I came back to it. :)

    AttributeError: 'NoneType' object has no attribute 'groups'

Upgrade github3.py and everything works again.
Add --since and --until

These flags make it feasible to run github-pr-stats on repos with large
numbers of pulls without having to plan ahead for many, many API calls
and a long run-time.  It also makes for a more useful management
analytics tool.
Update progress meter.

Handling --until required switching the order in which we get issues
from github.  Unfortunately, that means we can no longer estimate our
progress through the pulls (since we're counting forward and have no
idea how many total issues there are, and if using --until it'd be
thrown off anyways).

It's still nice to know that things are happening, though, so we'll now
print out the pull request number we're looking at.  And if you know
your repo, you can get a rough estimate of the progress, so I guess it's
ok.