Add ASViewController and update Multiplex sample to use it.

AsyncDisplayKit/ASViewController.h

@@ -0,0 +1,18 @@
+//
+//  ASViewController.h
+//  AsyncDisplayKit
+//
+//  Created by Huy Nguyen on 16/09/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import <UIKit/UIKit.h>
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+
+@interface ASViewController : UIViewController
+
+@property (nonatomic, strong, readonly) ASDisplayNode *node;
+
+- (instancetype)initWithNode:(ASDisplayNode *)node;
+
+@end