Use Factoryboy to generate data for unit tests (refactoring of test/unit/address/models_tests.py)

As previously discussed here https://groups.google.com/d/topic/django-oscar/Owmi3J1rwlk/discussion
I have started working on the introduction of Factoryboy library into django-oscar, to make the generation of data for unit tests easier. At the moment I have only refactored test/unit/address/models_tests.py, but the plan is to refactor all the existing tests. All the tests, of course, pass.