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:  we aren't technically adding a dependency or blocking node display blocks on each others' completion — maybe clarify that we're talking about display of an entire node hierarchy completing?