the regular Django dumpdata command now automatically works correctly
for polymorphic models and all Django versions
(this is handled by PolymorphicModelBase now).
Testing for dumpdata has been added.
- by default, django_polymorphic's pretty printing of querysets/objects (via ShowField*) is not used anymore
- ShowField mixins now also show the annotations (after the regular fields, prepended by "Ann:")
- cleaned up implementation.