Fix naming convetion:

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

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.h

@@ -0,0 +1,25 @@
+/*
+ *  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>
+
+/**
+ Lays out a single layoutable child, then lays out a background layoutable instance behind it stretched to its size.
+ */
+@interface ASBackgroundLayoutSpec : ASLayoutSpec
+
+/**
+ @param child A child that is laid out to determine the size of this spec. If this is nil, then this method
+        returns nil.
+ @param background A layoutable object that is laid out behind the child. May be nil, in which case the background is omitted.
+ */
++ (instancetype)newWithChild:(id<ASLayoutable>)child background:(id<ASLayoutable>)background;
+
+@end