Relocate Payflow dashboard package

This avoids an issue with Oscar's new dashboard permissions that require
the dashboard views to have the subpackage 'dashboard'.
Ensure compatibility with Django 1.6
Ensure digital orders get NoShippingRequired shipping method

This was triggered from a review of #55, which adds a commit to do the
same thing.  But since the core problem of #55 was already solved, it
was easier to just apply the change manually.