Simple UIScrollView wrapper node.

Closes #99.  This is a quick sketch -- room for improvement includes
considering a less-questionable class name, potentially exposing
additional UIScrollView functionality directly on the node, and using it
in a sample project.

AsyncDisplayKit/ASScrollNode.h

@@ -0,0 +1,23 @@
+/* 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/ASDisplayNode.h>
+
+#import <UIKit/UIKit.h>
+
+/**
+ * Simple node that wraps UIScrollView.
+ */
+@interface ASScrollNode : ASDisplayNode
+
+/**
+ * @abstract The node's UIScrollView.
+ */
+@property (nonatomic, readonly, strong) UIScrollView *view;
+
+@end