Add Twitter sentiment analysis

It'd be neat if we could add an extra widget to the dashboard that shows the average sentiment of Bitcoin-related tweets (in real-time). Maybe a gauge showing the ratio of positive to negative tweets.

We couldn't figure this out without having users register for a Twitter API key (which is a bit of a pain in the ass), and even then each user would be rate limited. But I think it would be useful to the users that factor in social media sentiment into their trading decisions.