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.
I think this may need to be in `-willDisplayAsyncLayer:` / `-displayWillStart` — right now, we'll only configure pending displays the first time a node enters a hierarchy, but display can be aborted and restarted without a node's hierarchy status changing