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
s/becode/because/ although, I'll just rephrase the doc comment like this:
```go
// fixMAAS restores /etc/network/interfaces file to the original format
// Ubuntu 14.04+ uses (i.e. one master interfaces file and one .cfg
// file per interface).
```