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.
setting this to `nil` should make ASImageNode's implementation of `-placeholderImageForSize:` return `nil` itself (but it shouldn't set `placeholderEnabled` to `NO`, in case a subclass wants to do something different)