Add support for managing large amounts of vouchers grouped in sets.

As in `test_model.py` -> `test_models.py`

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

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.

Use global instead of per-method test dates.

Required several schema changes:
* Add num_applications to offer model to track number of times an offer
  has been applied within an order.
* Add max_global_applications to offer model to set a limit on how many times an offer
  can be used.
* Add frequency to order discount to provide insight into how many times
  an offer was applied for a particular order.

Structure of dashboard changes to talk about offer 'availability'
instead of assuming a start- and end-date.

Also added tests for vouchers

Plus tweaks to test method names

Also split them into unit and functional tests.