Improve README.
Switch FACTORY_FOR and related to 'class Meta'.

This is easier to declare, avoids cluttering the namespace, and provides
entry points for ORM-specific customization.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>
factory.django: Fix counter inheritance with abstract models.
Migrate factory.alchemy to class Meta
Add tests for class Meta
Switch tests to class Meta.
Remove containers.DeclarationsDict.

Now replaced with a simple dict.
Add DeprecationWarning for FACTORY_* kwargs
Update ChangeLog
Update docs for class Meta.