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/ASDisplayNode.h

@@ -559,14 +559,6 @@ typedef void (^ASDisplayNodeDidLoadBlock)(ASDisplayNode *node);
  * 
  * If this node was measured, calling this method triggers an internal relayout: the calculated layout is invalidated,
  * and the supernode is notified or (if this node is the root one) a full measurement pass is executed using the old constrained size.
- * 
- * Note: If the relayout causes a change in size of the root node that is attached to a container view, 
- * the container view must be notified to relayout. 
- * For ASTableView and ASCollectionView, instead of calling this method directly, 
- * it is recommended to call -relayoutRowAtIndexPath:withRowAnimation and -relayoutItemAtIndexPath: respectively.
- *
- * @see [ASTableView relayoutRowAtIndexPath:withRowAnimation:]
- * @see [ASCollectionView relayoutItemAtIndexPath:]
  */
 - (void)setNeedsLayout;