IndexMixin inheritance issue

When using e.g. `ACLIndexMixin` like this:

``` js
var MyIndexCollection = MyBaseCollection.extend(
  _.extend({}, ACLIndexMixin, {
  ...
  })
);
```

Functions defined in `ACLIndexMixin` will overwrite functions defined in `MyBaseCollection`. This behaviour might be unwanted, so there should be someway of retaining behaviour form `MyBaseCollection`.
