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.
I went with the latter...if `sock.Born` weren't referenced by `Worker.sniffer()`, it might have made sense to remove it entirely.