Auto-import *.serializers to catch 'em all.

Without this, one could easily fall into the trap of not importing a
serializer, leading to the use of an automatic serializer instead, which
is definitely undesirable.
Redo the serializers technique.

Notable changes:

- ``Meta.fields`` now behaves more like it does in Django REST framework
  than how it has been: if you specify ``Meta.fields``, then manually
  defined fields *are not included by default any more*. You must add
  them to the serializer.

- ``Meta.exclude``, ``Meta.fields``, &c. now apply to newly added fields
  only. That is, you can't override the fields that will be displayed
  from parent CQRS classes. Worth while noting here is that fields added
  from abstract classes (CQRS or otherwise) are considered new fields.
  An abstract class cannot have a serializer at present.

Things should now be more stable and reliable and reasonable.

Much better tests of various non-polymorphic parts are also added, but I
fear the tests as a whole are still too closely tied to the
``Meta.fields`` approach rather than the ``Meta.exclude`` approach,
which should now be preferred.