Add a User-Agent header to prevent 403 API responses and close #6

I've added a simple User-Agent with the current version number of the GitHubot package. 

This may need to be updated with a customer User-Agent per install or per user so that the requests don't get rate limited all in the same bucket. http://developer.github.com/changes/2012-10-14-rate-limit-changes/

This is my first pull request on a node project, so please don't be gentle. Tear it apart if it sucks or does something the "wrong" way. :)