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
I don't see why we're using a StringsWatcher at all then. If we're just implementing a throwaway single-use worker, why not *write* a throwaway single-use worker, rather than misusing a framework type that's not going to help solve the problem in the long term?

But... are you *sure* we need the machine's lifecycle etc in here? My conception was that all we need here is to watch requested networks, and it's the api server's responsibility to tell us what those are -- making the this worker know or care about services or units (or probably machines for that matter) STM to be Doing It Wrong).

But, really, if we're not using a watcher, don't use a framework type that depends on a watcher. Just write a type with a no-op Stop and a Wait that just waits for the single DoSomething method to finish.