Error tracking

[technicalpickles]

While working on #2 I had a few times Hubot was throwing an error, but I didn't have anyway to see it.

We're hosted on Heroku, and there's a number of addons for it. I was thinking to go with Rollbar because it has a free tier, and also has GitHub Integration.

Part of this would be writing error handler for hubot. There aren't any open source plugins for error handlers that I know of, so that would be great to release as it's own package too.