[patch] [test] sqlalchemy.sql.visitors.Visitable descendants should define __visit_name__ excplicitly and this property should be inheritable

[Former user]

Discussion starts from here: http://groups.google.com/group/sqlalchemy/browse_thread/thread/85f275e5d7b08ced/96945c2607909e24

I made some automated tests to make sure that nothing will break. Existing Visitable's descendants will have exactly the same value of __visit_name__ property after applying the patch. I also put small test to ensure that now it is safe to extend sqlalchemy.schema.Column. It fails with vanila trunk and pass ok with patched.

[Mike Bayer]

adapted the test to be more specific, patch applied in b22edf1d8a16467a8d5625f4ddf7dca9403bdfe0..thanks !