Support memoization

In my mind, denormalizr is most useful when passing large nested objects to a component using connect. But currently it always returns a new object, so the component re-renders on every action.

denormalize should always return the same (strictly equal) result unless:

1. The root object is different
2. Any of the associations in the schema are different

I believe this could be accomplished fairly simply using a cache of denormalized resources.