Create a factory module per app and move them under the new
oscar.test.factories module.

Applicator.get_basket_offers() checks for applicable basket offers using
something like if voucher.is_available_to_user(user) (see
https://github.com/django-oscar/django-oscar/blob/master/src/oscar/apps/offer/utils.py#L99).
However, the return value of is_available_to_user() is a 2-tuple, so
this will always evaluate to True.

Closes #1647

Oddly enough, the warnings were only thrown under Python 2.6. Should
investigate!

Using timezone.now() will return naive datetimes if USE_TZ is not
enabled, which is what we want.
Resolves errors of comparing naive datetimes with aware datetimes.

The clean method was raising a KeyError when the date fields were empty.