Use Marionette.CollectionView in MultiSelect

We could make use of [`Marionette.CollectionView`](https://github.com/marionettejs/backbone.marionette/blob/master/docs/marionette.collectionview.md) when rendering the MultiSelect lists. Together with #1 this would improve the possibilities of managing MultiSelect lists, allowing one to bind events between a model and a view directly.
