Adds AWPagedArrayController

Hello!

I liked the functionality of the DataProvider from the demo app and wanted to generalize it so it could be used in a variety of contexts, so I created the AWPagedArrayController. It functions very similarly to the DataProvider, but uses an arbitrary block with a completion handler to load pages. I took the liberty of changing the DataProvider to be a subclass of the AWPagedArrayController as a demonstration.

To prevent the AWPagedArrayController from having it's backing AWPagedArray mutated, I added the NSCopying protocol to the AWPagedArray class.

I'd love to hear your thoughts on these additions!