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.
Hm, that's a good point.  Could we build the rendering-subnodes sets from the bottom up, with display-enabled placeholder-less children notifying their parents that they're rendering?  Hierarchy-change notification semantics don't correspond well to displays, and they can even be disabled without warning (`ASDisplayNodeDisableHierarchyNotifications()`), so they're unsafe to rely on.