WIP: networker skeleton

worker: networker skeleton

This RP is a skeleton for the future networker based on StringsWorker

Continue of work from: https://github.com/juju/juju/pull/86
Please explain what these names are going to indicate. AFAICS you've written the handler code for a NotifyWatcher in Setup, and if you just moved that in here you'd be nearly done; but the current approach is going to require a major rewrite as soon as strings are sent down.