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.
yup, `[super dealloc]` should automatically be inserted by ARC in custom `-dealloc` implementations.  this is fine, just spitballing potential alternatives — since we don't need strong references here, good to keep them in mind =]