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 adviced Vlad to put all this in the SetUp method, as we'll have to handle other cases in the Handle method. Effectively, we need to watch several things and handle the changes in one place - subnets life cycle, machine life cycle, raw addresses, and requested networks (machine's and all deployed services').