Try to require only one of tweepy and python-twitter

[myersjustinc]

We have both tweepy and python-twitter listed as dependencies right now, so we should investigate why each is being used and attempt to standardize on one or the other. I think python-twitter is used for pre-OAuth code and tweepy is used for post-OAuth code, so I imagine tweepy's the way to go--but Esten's probably the guy to ask on this.