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.
this still makes me pretty uneasy — could we have nodes add themselves to their supernode's `_pendingDisplayNodes` with a `-displayWillStart` callback?