feat(metrics): Add github.com/rcrowley/go-metrics
feat(metrics): Add metrics pkg
feat(metrics): Add documentation and contrib scripts
feat(metrics): enable some metrics; push to graphite

* -trace flag controls whether or not to enable metrics-gathering
  and the /debug/* HTTP endpoints
* -graphite-host flag controls where metrics should be sent
* timer.ae.handle metric tracks execution time of AppendEntriesRequest
DO NOT MERGE

feat(metrics): emit HeartbeatDuration event
feat(metrics): Publish peer heartbeat events as metrics