Add tox configuration file.

Note: The tests do not currently pass.
Update DB settings for Django >= 1.4.
Import settings in tests from django.conf.
Update db_type method for Django >= 1.4 DB settings format.
Fixed the default manager test failures.

According to the django docs (1.4 / 1.5), the default manager for subclasses of
PolymorphicModel should be PolymorphicManager:

3. The default manager on a class is either the first manager declared on the
   class, if that exists, or the default manager of the first abstract base
   class in the parent hierarchy, if that exists. If no default manager is
   explicitly declared, Django’s normal default manager is used.

https://docs.djangoproject.com/en/1.4/topics/db/managers/#custom-managers-and-model-inheritance
https://docs.djangoproject.com/en/1.5/topics/db/managers/#custom-managers-and-model-inheritance