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 method is called by `-_pendingNodeDidDisplay:`, which already has `_propertyLock` — won't this explode, or am I misreading something?  =]