-[UIView addSubnode:], -[CALayer addSubnode:].

Bring back this convenience API -- it disappeared somewhere along the
line while we were building Paper.  This is totally trivial, but
conveniently won't break if you layer-back a leaf node.

Closes #278.

examples/Multiplex/Sample/ViewController.m

@@ -61,7 +61,7 @@
 {
   [super viewDidLoad];
 
-  [self.view addSubview:_imageNode.view];
+  [self.view addSubnode:_imageNode];
   [self.view addSubview:_textLabel];
 
   [self start];