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.
nit:  you can use `self.placeholderEnabled` instead of `self.isPlaceholderEnabled` — the `is…` property accesses are leftovers from a couple of codemods