Prior to this change, it was possible to select child products in the
product dashboard. A new function get_parent_products_queryset was
introduced, analogues to the existing get_related_products_queryset. It
is not based on the BrowsableManager as I can envision scenarios where
that is extended to respect status fields or such like, which we should
ignore when selecting a parent field. Otherwise it wouldn't be possible
to e.g. add a few hidden products with child products.

Two issues in tests with the G library were fixed as well, were
unintentionally child products were created. If the product field is
left unspecified, it seems to pick a product instead of leaving it as
None.

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

Fixes #376

The previous UPC validation in ``ProductForm`` broke updating the
a product. I realised this while digging around to fix the broken
``StockRecord`` issue that broke the CI build.

The validation now uses Django's validation of ``unique`` fields
instead and works.

The ``StockRecord`` issue was related to creating a form in the
update view providing stock record data in POST with ``partner``
empty. I corrected that and added a test case for it.

Attempting to create a new product with a UPC that already exists
raises an ``IntegrityError`` instead of handling the ``ProductForm``
as an invalid form with an appropriate ``ValidationError``.

I fixed that by adding a ``clean_upc`` validation method to the
form by checking for an existing product by UPC. I am not sure if
that is the most efficient way to validate this. If there's a
better alternative, I am happy to change it...and learn something
:)

**Note:** I switched the functional tests in ``product_tests.py``
to use ``WebTest`` while I was at it.

The validation on the stockrecord form needed to be removed to allow
group products to be created, which don't have stock records.