Bring back this convenience API -- it disappeared somewhere along the
line while we were building Paper. This is totally trivial, but
conveniently won't break if you layer-back a leaf node.
Closes#278.
ASDisplayNodes now have an overidable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asyncronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.
