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:  feel free to make this warning more loud!  see above—

>   `* @note Called on the display queue and/or main queue (MUST BE THREAD SAFE)`