Added ASTextNodeWordKerner support to ASTextNode

* Added a member to `ASTextKitAttributes` that is an optional delegate to the struct's NSLayoutManager.
* Changed ASTextNode to set this delegate to an instance of ASTextNodeWordKerner.
* Updated init method of `ASTextKitContext` to take an optional NSLayoutManager delegate
* Added the files in TextKit folder to the public headers (so we can include ASTextNodeTypes.h)

AsyncDisplayKit/TextKit/ASTextKitAttributes.h

@@ -89,6 +89,11 @@ struct ASTextKitAttributes {
    A pointer to a function that that returns a custom layout manager subclass. If nil, defaults to NSLayoutManager.
    */
   NSLayoutManager *(*layoutManagerFactory)(void);
+  
+  /**
+   An optional delegate for the NSLayoutManager
+   */
+  id<NSLayoutManagerDelegate> layoutManagerDelegate;
 
   /**
    We provide an explicit copy function so we can use aggregate initializer syntax while providing copy semantics for