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.
shouldn't we reset `_pendingDisplays` when this method starts?  in the current hierarchy-notification-based system, if a node's view is moved between UIWindows while it's rendering, its pending display count will be doubled and its placeholder will never go away.  if we move `_setup` to `-displayWillStart`, matching the paired teardown call in `-displayDidFinish`, then suspending and restarting display on a node will probably have a similar effect.

on reflection, it may be simplest to maintain an actual set of rendering subnodes.  thoughts?