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.
:frowning: rookie mistake! absolutely should be reset.

I'm going to refactor to strictly use a NSMutableSet. Maybe down the road once everything is ironed out we can go back to using a count & keep it simple, but at the moment it'll be rough to debug.