Ajax selects for dashboard's product fields

This should greatly increase usability of the dashboard product update
page when managing a large amount of products.

Fixes #618

Conflicts:
	oscar/apps/dashboard/catalogue/forms.py
	oscar/apps/dashboard/catalogue/views.py
	oscar/static/oscar/js/oscar/dashboard.js
	oscar/views/generic.py

Widgets for product selection using select2 and ajax