Add ASViewController and update Multiplex sample to use it.

examples/Multiplex/Sample/ScreenNode.h

@@ -0,0 +1,19 @@
+//
+//  ScreenNode.h
+//  Sample
+//
+//  Created by Huy Nguyen on 16/09/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+
+@interface ScreenNode : ASDisplayNode
+
+@property (nonatomic, strong) ASMultiplexImageNode *imageNode;
+@property (nonatomic, strong) ASTextNode *textNode;
+
+- (void)start;
+- (void)reload;
+
+@end