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.
Took me a few seconds to see how the timer became this. I'd suggest eitheruse timer here instead of sock.Born *or* when making PushWS, just say Born: time.Now() rather than copy the timer around.