cant subclass TypeDecorator types

[Mike Bayer]

a user is trying to subclass Unicode and having endless loop problems. this is probably because Unicode is already a subclass of TypeDecorator. the Type -inheritance system should be looked into more closely, and should probably use a more composed structure between the generic types and the DB-specific types so that they can be more easily extended. so much for python magic !

[Mike Bayer]

I just did a rework of types as above. the unittest has a user-defined subclass of String as well as a user-defined subclass of Unicode, both work. all tests pass so I am closing this for now, will re-open if problems persist. TypeDecorator is a no-op now. changeset changeset:1121