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/__init__.py

@@ -0,0 +1,2 @@
+from .all_warnings import AllWarnings
+from .document import *