Don't render hidden fields twice.

Especially not with the visible label that partials/form_field.html outputs.
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 'product-class-use-slug-in-URL' into machtfit

* branch product-class-use-slug-in-URL:
  Use slug instead of id for product_class in URL
  Add IRC channel to README
Merge branch 'fix-visible-hidden-field' into machtfit

* branch fix-visible-hidden-field:
  Don't render hidden fields twice.
Bump version.