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:  maybe mention that we generate placeholders at `-measure:`-time so that a node is guaranteed to have a placeholder ready to go as soon as it's been sized?