Watch $scope.$index and reloadItems if the position of an item has changed

This adds reordering support using orderBy filter for example. :-)

```html
  <div id="container" masonry>
    <div class="masonry-brick" ng-repeat="brick in bricks | orderBy:predicate:reverse">
       <img ng-src="{{ brick.src }}" alt="A masonry brick">
    </div>
  </div>
```