Switch to using freezegun for tests that depend on date.

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.

The tests check if the year 2010 is a valid option but since only the
last 5 years are rendered this is no longer the case. For now I've
changed the tests to check if 2014 is valid which should be the case the
next few years. A better option is to use the `freezegun` module in the
future.

TestCase.assertEquals (note the s at the end) has been deprecated.
The only failing tests remaining are CSV related, which is not
surprising.

Fix to allow users to specify which cards they accept.
By default, this works exactly as it did- i.e., any number
can be entered and only a luhn check is run on it.
Users can now overide this behaviour by passing a list of
accepted cards as a 'types'kwarg to BankcardNumberField,
e.g.
BankcardNumberField(types=[bankcards.VISA, bankcards.VISA_ELECTRON,])

If this is specified, we check that the type of card is one we're
aware of and then that the card number matches the pattern for that
type of card

Fixes #949

@maikhoepfel rewrote the ImproperlyConfigured check using Python's sets.

Conflicts:
	oscar/apps/payment/forms.py

Fields now more self-contained, the bankcard form is much simpler.