Add backkwards-compatible Bankcard class factory

This provides a way for extensions that need to work with Oscar 0.5 and
0.6 to consistently load the Banckard class (even though it was moved
and had its API changed between versions).
Upgrade demo site to use new Datacash release
Correct erroneous use of fetch() in basket form
Merge branch 'easily-add-new-formset-to-product-update-page' into machtfit

* branch easily-add-new-formset-to-product-update-page:
  Provide a hook for cross-form validation
  Make it trivially easy to add more formsets to the product update page
  Unify __init__ method signature of formset classes
  customize FormSet class after calling inlineformset_factory

https://github.com/tangentlabs/django-oscar/pull/1022
Merge branch 'passwordreset-singleobjectmixin' into machtfit

* branch passwordreset-singleobjectmixin:
  Make it easy to restrict the users you can use this form with.

https://github.com/tangentlabs/django-oscar/pull/1052