add a etcd monitor

[felipejfc]

This monitor is cleaner than redisMonitor because of how etcd works, using watchers is just simpler.
There should be a big improvement in networking usage on big clusters using this monitor because it only traffic data when it's necessary.
The etcd lib used already supports specifying many etcd nodes(for clustered etcd setup) and automatically handle reconnects.

Example usage

app.configure('development', function() {
  app.set('monitorConfig', 
    {
      monitor: pomelo.monitors.etcdmonitor,
      etcdNodes: ["localhost:2379","localhost:2389","localhost:2399"],
      period: 2000,
      prefix: "pomelo-1"
    })
})

New constants

DEFAULT_ETCD_PERIOD
The default period that the servers will use to refresh ttl on their keys @ etcd.

[cynron]

awesome! thanks!