Only allow addresse in valid ship-to countries to be shown at checkout

Before this a user could create an address in their account that isn't
in a shipping country, and it would still be available to ship to in the
checkout.  This changes ensures that only countries that are marked for
shipping can be selected at checkout.
Adjust address list view for clarity
Handle non-namespaced URL names in dashboard nav

This completes the change in #967 and ensures that Oscar sites can use
non-namespaced URLs in the dashboard.
Upgrade datacash extension for demo site
#871 default gallery slightly better looking, not need for absolutes
Merge branch 'master' of https://github.com/tangentlabs/django-oscar into issue/871/carousels