Changed the inheritance model to remove types

The inheritance model has changed, we no longer need to store an array of
`types` with the model we can just use the classname in `_cls`.

See the upgrade docs for information on how to upgrade
MongoEngine/mongoengine#148

tests/migration/__init__.py

@@ -0,0 +1,4 @@
+from turn_off_inheritance import *
+
+if __name__ == '__main__':
+    unittest.main()