Adjusting add/remove event handlers to not re-render entire collectionView

I've refactored the code that creates and adds a model's view to the collectionView into the private function _renderModelView.  

I've also added the method to look up a model's view in the viewManager and remove it.

These two methods are used by the listeners on collection.add and collection.remove to update the collectionView without a full re-render.

This should hopefully address the issue discussed here: https://github.com/rotundasoftware/backbone.collectionView/issues/32

I've tested against sorted collectionView's and the collections are indeed sorted properly.

If you have any questions or suggestions I'm all ears.

-Elliott