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/ASTextKitContext.mm

@@ -28,6 +28,7 @@
                           exclusionPaths:(NSArray *)exclusionPaths
                          constrainedSize:(CGSize)constrainedSize
                     layoutManagerFactory:(NSLayoutManager*(*)(void))layoutManagerFactory
+                   layoutManagerDelegate:(id<NSLayoutManagerDelegate>)layoutManagerDelegate
 {
   if (self = [super init]) {
     // Concurrently initialising TextKit components crashes (rdar://18448377) so we use a global lock.
@@ -37,6 +38,7 @@
     _textStorage = (attributedString ? [[NSTextStorage alloc] initWithAttributedString:attributedString] : [[NSTextStorage alloc] init]);
     _layoutManager = layoutManagerFactory ? layoutManagerFactory() : [[NSLayoutManager alloc] init];
     _layoutManager.usesFontLeading = NO;
+    _layoutManager.delegate = layoutManagerDelegate;
     [_textStorage addLayoutManager:_layoutManager];
     _textContainer = [[NSTextContainer alloc] initWithSize:constrainedSize];
     // We want the text laid out up to the very edges of the container.