Migrate to express from restify

PeerServer is now instance of express application - this allows to combine it with other express applications.
Listen is deferred - because it now method from express (you can call it later and supply callback for start event).
Constructor now not using `new` (because we mixing in methods to express app)

Closes #36
Remove `return next()` - they are unnecessary in express
Pass `server` or `port` option to PeerServer

This allows to initialize websocket server properly
Tests with _server startup and teardown callbacks