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
Bump version.