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.
let's set `_placeholderLayer.zPosition` to something arbitrarily high, to make sure normal hierarchy manipulations don't inadvertently cover the placeholder and make everything look weird =]