Add facility for receiving strings and maintaining frequency counts.

There's no place for these string counts in graphite, so they're only maintained in tallier's memory. To view the top strings for a particular stat I've added a command-line interface over tcp. I plan on eventually wrapping this with an http server for the dashboard app to poll.

With this change, apps can be modified to start sending in string data. For now, tallier will automatically count all the stat keys it sees, so we can determine if there are any frequent useless stats we can remove to recover some bandwidth.