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.
ooh, that's a good point!  maybe we need to add a new hook ("subnodes will display"?) in the +AsyncDisplay machinery to support this robustly