Add active layout methods.
Only ask delegates for layout information if we're not on the paging layout.
Override setDelegate so that the collection view properly recaches implemented selectors.
Store the active layout as an ivar.

The collection view doesn't update its collectionViewLayout property
immediately when calling setLayout, so we track it ourselves.
Add collectionViewLayoutDidChange subclass hook.
Minor docs change.
Delete unnecessary subclass methods and expose which UIScrollViewDelegate methods are implemented.
Add __NI_REQUIRES_SUPER for UIScrollViewDelegate methods.