Social API port event listener/dispatcher

This PR refactors the previous `app.services` object to a new `app.port` one to directly use the `Backbone.Events` interface it already implemented, thus allowing more conveniently to:

- listen to worker events and react accordingly
- trigger internal events
- post events to the worker

Typical usage:

```js
// posts an event to the worker
app.port.postEvent("talkilla.topic", {foo: "bar"});

// listens to a worker event
app.port.on("talkilla.topic", function(data) {
  console.log(data);
});
```

I suggest a review from @Standard8 as we already discussed this while pair programming, but any feedback is warmly welcome.

**Edit:** pair-reviewed and discussed with @tOkeshu