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.
The reason I wanted to bucket the subnodes up front was in the off chance that a subnode actually finishes display before all subnodes are done notifying, resulting in the parent seeing an empty set of displaying nodes, then prematurely removing the placeholder. Probably not a frequent scenario, but w/ all of the async behavior couldn't it happen?