The variable chats are not implemented correctly in services

The variable chats are not implemented correctly in `services.js`.

See the file:
https://github.com/mappmechanic/ionic-firebase/blob/master/www/js/services.js#L13

change:

``` javascript
var chats;
```

to:

``` javascript
var chats = $firebaseArray(ref.child('chats'));
```

and has a guaranteed solution! :)
