Returning a channel from Pusher.subscribe

Hey! On the [client-event#trigger-events][trigger] section of the
docs, triggering client events is suggested to look like

```javascript
var pusher = new Pusher('YOUR_APP_KEY');
var channel = pusher.subscribe('private-channel');
channel.bind('pusher:subscription_succeeded', function() {
  var triggered = channel.trigger('client-someeventname', { your: data });
});
```

But, this doesn't work in this angular-pusher library, because this
lib's `Pusher.subscribe` [doesn't return anything][return]. How do you
want to get the channels back out of this library's Pusher.subscribe
method? It's all taking place in a promise, so it seemed a bit
complicated to me to get just a channel object out. 

As a kludge, I put the currently subscribed channel
[on the Pusher factory itself][mine], but that doesn't generalize to
multiple channels, and the interface is different from the normal JS
library. I couldn't figure out a way to mirror the normal interface
because of the promise, so I'm curious if y'all have already thought
about this.

[trigger]: http://pusher.com/docs/client_api_guide/client_events#trigger-events
[return]: https://github.com/doowb/angular-pusher/blob/gh-pages/angular-pusher.js#L89-L98
[mine]: https://github.com/gempesaw/honeydew-ng/blob/master/app/bower_components/angular-pusher/angular-pusher.js#L92