Allow polymorphic embedding of types

> Allow multiple types to be embedded in a single embedList field.
> 
> This could be achieved either by allowing the embeddedList to hold multiple types or to introduce a type hierarchy system so that types can inherit from other. The former easier and probably preferred.
> 
> See this discussion: http://groups.google.com/group/kanso/browse_thread/thread/907e638a99390109

From: kanso/kanso#136
