Clean up #468:

- -layoutSpecThatFits: must return an ASLayoutSpec.
- Move ASDisplayNode's -measureWithSizeRange: redeclaration to ASDisplayNode.h.
- Rename ASStackLayoutChild.h to ASStackLayoutDefines.h.
- Rename ASStaticLayoutSpecDimension.h to ASRelativeSize.h.
- Don't import ASLayout.h in other headers to prevent circular inclusions.
- Explain use cases of ASLayout's initializers.
- Clean up ASInternalHelpers.h.

AsyncDisplayKit/ASDisplayNode.h

@@ -13,6 +13,8 @@
 #import <AsyncDisplayKit/ASDealloc2MainObject.h>
 #import <AsyncDisplayKit/ASDimension.h>
 
+#import <AsyncDisplayKit/ASLayoutable.h>
+
 typedef UIView *(^ASDisplayNodeViewBlock)();
 typedef CALayer *(^ASDisplayNodeLayerBlock)();
 
@@ -32,7 +34,7 @@ typedef CALayer *(^ASDisplayNodeLayerBlock)();
  *
  */
 
-@interface ASDisplayNode : ASDealloc2MainObject
+@interface ASDisplayNode : ASDealloc2MainObject <ASLayoutable>
 
 
 /** @name Initializing a node object */
@@ -137,6 +139,23 @@ typedef CALayer *(^ASDisplayNodeLayerBlock)();
  */
 - (CGSize)measure:(CGSize)constrainedSize;
 
+/**
+ * @abstract Asks the node to measure 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, if the box layout model is used).
+ *
+ * @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 -calculateLayoutThatFits:.  Calling this method may
+ * be expensive if result is not cached.
+ *
+ * @see [ASDisplayNode(Subclassing) calculateLayoutThatFits:]
+ */
+- (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize;
+
 /** 
  * @abstract Return the calculated size.
  *