When doing refresh, we should cancel perform requests previously registered with performSelector:withObject:afterDelay: first, or it will do the sizeNextBlock twice with the same indexPath and then it will crash.
Merge pull request #147 from yufenglv/master

fix the crash when calling rebuildData (#138)
Node placeholder API

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.
Updating placeholder specs
Text node line placeholders and documentation
Subnode placeholder API changes

Text node line placeholders and documentation