Split cqrs.mongo into appropriate modules.

These are:

- cqrs.models
- cqrs.settings
- cqrs.serializers
- cqrs.mongo (the Mongo-specific parts stay there)
merge
commented out placeholder for the get_related_models
Merge branch 'automatic-polymorphic-serializer' of ssh://github.com/commoncode/cqrs into automatic-polymorphic-serializer