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.
It would, but doesn't ```_pendingDisplayNodes = nil``` in ```-dealloc``` solve this? It would definitely require calling ```[super dealloc]``` from subclasses, but that should be required already.