Implemented Firebase priority ordering, no more extra attribute

I implemented the priority ordering described here:
https://www.firebase.com/docs/javascript/firebase/setpriority.html

This way an angularFireCollection's ordering is completely consistent with that of Firebase's. The reason for this implementation is to use angular's orderBy filter. Below is an example usage, given an angularFireCollection named 'widgets':

```html
<div ng-repeat="widget in widgets | orderBy:widgets.order">
    {{widget.name}}
</div>
```

It uses the 'Array of predicates' flavor of expression, see:
http://docs.angularjs.org/api/ng.filter:orderBy#Parameters