Allow ProductAttributes to be edited from the ProductClass form in the dashboard.

Summary:
* Ui has tabbed navigation like the product edit form.
* The option_group select field is hidden unless you select `type=option`

In the first commit, the `ProductClassCreateView` and `ProductClassUpdateView` have been merged into `ProductClassCreateUpdateView` but this may be incompatible with sites that expect the Create and Update views to exist separately, in the 2nd commit `ProductClassCreateView` have been returned `ProductClassUpdateView`. There might still be some issues for people using the stock form since it now expects `attributes_formset`.

Finally if this gets approved, I can get started on the ui for editing ProductAttributeOptions when creating/editing ProductAttributes. Following [this discussion](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!topic/django-oscar/-LmhwpAxgaU), I believe implementing as many features as possible within the dashboard would really help oscar get a larger community.