Fix naming convetion:

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

AsyncDisplayKit/Layout/ASLayoutSpec.h

@@ -0,0 +1,17 @@
+/*
+ *  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/ASLayoutable.h>
+#import <AsyncDisplayKit/ASLayout.h>
+
+/** A layout spec is an immutable object that describes a layout, loosely inspired by React. */
+@interface ASLayoutSpec : NSObject <ASLayoutable>
+
+@end