Exponential backoff + watcher retry

Suggested fix for https://github.com/stianeikeland/node-etcd/issues/28

If a watcher receives an unexpected / invalid response, it will now go to _retry, instead of instantly asking again. This to avoid ddos attacking ourselves.

I've also added an exponential backoff with a random factor to it so we won't crash ourselves by having hundreds of services reconnect at the exact same time.