Node placeholder API

ASDisplayNodes now have an overridable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asynchronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.
hmm, doesn't the `&& _placeholderLayer` prevent placeholders from being generated if `-measure:` is called before the node's view has been loaded?