Limit block and page types to be created only once (updated)

Two years ago I did a hack that let you as a developer specify if a block or page type should be allowed to only be created once. Old post here. For some reason I can't remeber I used an interface to specify this. Now I have updated the code to use an attribute instead. 

Just decorate your models with [Singleton] and the initializationmodule will handle the rest.

Code over at Gist.