Migrate changes to include updating single elements of ListFields as
well as MapFields by adding the same changes to BaseList. This is
done by ensuring all BaseDicts and BaseLists have the correct name
from the base of the nearest (Embedded)Document, then marking changes
with their key or index when they are changed.
Tests also all fixed up.
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
