Test different Django versions

As Oscar supports multiple Django versions, the extension really should
be tested against all of the supported ones. This will get really fun
when we add support for Python 3!
Remove unused get_model call

This call causes issues with Django 1.7 as it's called during the app
initialisation sequence. Luckily, it's not needed anyway, and can be
removed.
Conditionally add South to INSTALLED_APPS

South is only supported in Django < 1.7, so it's only added when needed.
Use Django 1.7 enabled django-nose

django-nose has not been released with Django 1.7 support yet, so we
need to run against a custom but working version. The same thing is
needed in Oscar.
Use line instead of stockrecord to determine price

Product.stockrecord was a deprecated helper to get the first stockrecord
for a product, so it was potentially the wrong stockrecord to look at
anyway. Using the price stored on the basket line is the correct thing
to do, and avoids the AttributeError as Product.stockrecord has since
been removed.
Explicitly set MIDDLEWARE_CLASSES

Django 1.7 removed a few middleware classes from the default setting,
but Oscar relies on them being available.
Bump version to 0.8.2