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.

examples/Kittens/Sample/KittenNode.mm

@@ -185,6 +185,7 @@ static const CGFloat kInnerPadding = 10.0f;
 - (void)toggleImageEnlargement
 {
   _isImageEnlarged = !_isImageEnlarged;
+  [self setNeedsLayout];
 }
 
 - (void)toggleNodesSwap

examples/Kittens/Sample/ViewController.m

@@ -121,7 +121,6 @@ static const NSInteger kMaxLitterSize = 100;        // max number of kitten cell
   // Assume only kitten nodes are selectable (see -tableView:shouldHighlightRowAtIndexPath:).
   KittenNode *node = (KittenNode *)[_tableView nodeForRowAtIndexPath:indexPath];
   [node toggleImageEnlargement];
-  [_tableView relayoutRowAtIndexPath:indexPath withRowAnimation:UITableViewRowAnimationAutomatic];
 }
 
 - (ASCellNode *)tableView:(ASTableView *)tableView nodeForRowAtIndexPath:(NSIndexPath *)indexPath