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.
`-placeholderImage` is something only node subclasses should know about (and implement), so it should stay in this file.  `-placeholderEnabled` is an option anyone can change on any node, so it should live in ASDisplayNode.h =]