Use slug instead of id for product_class in URL

This gives more readable URLs.
customize FormSet class after calling inlineformset_factory

That way there's no need to know what base class inlineformset_factory
uses and it's consistent with StockRecordFormSet.
Unify __init__ method signature of formset classes

so that we can treat them all the same in ProductCreateUpdateView. Also
add user argument as that will be needed for
https://github.com/tangentlabs/django-oscar/pull/1002
Make it trivially easy to add more formsets to the product update page
Provide a hook for cross-form validation
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
Bump version.