Integrate etcd and consul watch

I took a quick stab at integrating etcd and consul's watch features per issue #56. This update basically wraps the go-etcd `Watch` command and consul's `WatchList` so they both have the same interface (go-etcd uses goroutines/channels while consul is a long-poll).

It adds a `-watch` flag that takes precedence over any intervals:
`confd -watch -node 'http://127.0.0.1:4001' -confdir ~/confd`