Made it possible to create apps without a models module.

This commit reverts f44c4a5d0f and 39bbd165.

django.test.simple will be updated in a separate commit as it requires
invasive changes.
Refactored old test runner to handle apps without a models module.
Fixed incorrect decrementation of nesting_level.

Thanks Florian for catching this mistake.
Inlined trivial method that was used only once.
Restored deprecated aliases for functions in django.db.models.

Thanks Marc Tamlyn for the suggestion.