Introduce ASLayoutable and eliminate ASCompositeNode:

- Both ASDisplayNode and ASLayoutNode conforms to this protocol.
- ASDisplayNode can be embeded directly into layout graph.
- Eliminate ASCompositeNode.
- Fix ASStaticSizeDisplayNode not recpect min constrained size.
- Updated tests.

AsyncDisplayKit/ASDisplayNode.h

@@ -11,6 +11,7 @@
 #import <AsyncDisplayKit/_ASAsyncTransactionContainer.h>
 #import <AsyncDisplayKit/ASBaseDefines.h>
 #import <AsyncDisplayKit/ASDealloc2MainObject.h>
+#import <AsyncDisplayKit/ASLayoutable.h>
 
 
 typedef UIView *(^ASDisplayNodeViewBlock)();
@@ -32,7 +33,7 @@ typedef CALayer *(^ASDisplayNodeLayerBlock)();
  *
  */
 
-@interface ASDisplayNode : ASDealloc2MainObject
+@interface ASDisplayNode : ASDealloc2MainObject <ASLayoutable>
 
 
 /** @name Initializing a node object */
@@ -128,10 +129,10 @@ typedef CALayer *(^ASDisplayNodeLayerBlock)();
  * @discussion Though this method does not set the bounds of the view, it does have side effects--caching both the 
  * constraint and the result.
  *
- * @warning Subclasses must not override this; it caches results from -layoutNodeThatFits:.  Calling this method may
+ * @warning Subclasses must not override this; it caches results from -calculateLayoutThatFits:.  Calling this method may
  * be expensive if result is not cached.
  *
- * @see [ASDisplayNode(Subclassing) layoutNodeThatFits:]
+ * @see [ASDisplayNode(Subclassing) calculateLayoutThatFits:]
  */
 - (CGSize)measure:(CGSize)constrainedSize;
 
@@ -150,10 +151,25 @@ typedef CALayer *(^ASDisplayNodeLayerBlock)();
 /** 
  * @abstract Return the constrained size used for calculating layout.
  *
- * @return The constrained size used by layoutNodeThatFits:.
+ * @return The maximum constrained size used by calculateLayoutThatFits:.
  */
 @property (nonatomic, readonly, assign) CGSize constrainedSizeForCalculatedLayout;
 
+/**
+ * @abstract Calculate a layout based on given size range.
+ *
+ * @param constrainedSize The minimum and maximum sizes the receiver should fit in.
+ *
+ * @return An ASLayout instance defining the layout of the receiver and its children.
+ *
+ * @discussion Subclasses that override should expect this method to be called on a non-main thread. The returned layout
+ * is cached by ASDisplayNode for quick access during -layout, via -calculatedSize. Other expensive work that needs to
+ * be done before display can be performed here, and using ivars to cache any valuable intermediate results is
+ * encouraged.
+ *
+ * @note This method should not be called directly outside of ASDisplayNode; use -measure: or -calculatedLayout instead.
+ */
+- (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize;
 
 /** @name Managing the nodes hierarchy */