Local Discovery

This adds a plugin that will discover, connect to, and replicate with other peers running scuttlebot on the local network. This makes doing a live demo in a cafe a breeze!

Currently, it just replicates with everyone who is running a scuttlebot instance on the same network. Of course, eventually we'll want to make this more selective... for example, checking if a peer is a friend (i.e. followed) or at least known (a friend follows them)

I have also added a method to the api `getLocal` so that it's possible to add a feature to phoenix to show local peers, so that following is easier.