RB.CollectionView makes automatic rendering of items from a collection

easy, but it has a limitation in that all items render to the

collection's main element. This prevents subclasses from having a more

complex element tree.
This updates RB.CollectionView to introduce a new $container

attribute, which is used for all rendering. It defauls to $el, but

subclasses or consumers can override it to point to another element in

the view.

Unit tests passed.
Successfully used this new attribute in an upcoming change.