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.
The biggest issue I ran into, and why I decided on ```__enterHierarchy```, is that if a node doesn't implement drawing/display then a lot of the ```willDisplay...``` methods aren't called b/c the node isn't actually doing anything (even if its children are). It's mostly b/c it has no [display block](https://github.com/facebook/AsyncDisplayKit/blob/master/AsyncDisplayKit/Private/ASDisplayNode%2BAsyncDisplay.mm#L319-321). And that really makes sense b/c the node isn't really in charge of displaying anything.