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
`defer statetesting.AssertStop(c, nw)` (or something like that). I'm not sure statetesting is the best place for it, move it somewhere better if you can think of one.