Clean up stats

This pull request cleans up `Metrics` and adds several new timers and gauges. I verified that the stats are sent to statsd...do we want to migrate to Heka's `StatsdInput` at some point?

* Track message routing responses and latencies.
* Add gauges for tracking the number of connected clients.
* Capture timing information as soon as possible.

Closes #41.
It would probably be good to know how long it takes us to fail routing an update.