Fix naming convetion:

- "Layout node" is changed to "layout spec".
- Update tests.
- Update Kittens sample.

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.h

@@ -0,0 +1,20 @@
+/*
+ *  Copyright (c) 2014-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#import <AsyncDisplayKit/ASLayoutSpec.h>
+
+/**
+ This layout spec lays out a single layoutable child and then overlays a layoutable object on top of it streched to its size
+ */
+@interface ASOverlayLayoutSpec : ASLayoutSpec
+
++ (instancetype)newWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)overlay;
+
+@end