Introduce ASCellNodeDelegate

- Cell node automatically notifies the delegate after a relayout (via -setNeedsLayout) that results in a new size. Confirming to ASCellNodeDelegate; ASTableView and ASCollectionView reload the calling cell upon notifications. These views automatically set themselves as delegate of every node.
- The result is that ASCellNode subclasses don't need to manually notify the containing view. Thus, `-relayoutItemAtIndexPath` and `-relayoutRowAtIndexPath` are removed.
- Kittens example is updated to reflect the change.

AsyncDisplayKit/ASCollectionView.h

@@ -223,16 +223,6 @@
  */
 - (void)reloadItemsAtIndexPaths:(NSArray *)indexPaths;
 
-/**
- * Relayouts the specified item.
- *
- * @param indexPath The index path identifying the item to relayout.
- *
- * @discussion This method must be called from the main thread. The relayout is excuted on main thread.
- * The node of the specified item must be updated to cause layout changes before this method is called.
- */
-- (void)relayoutItemAtIndexPath:(NSIndexPath *)indexPath;
-
 /**
  * Moves the item at a specified location to a destination location.
  *