I love dict expressions, but my server is stuck on 2.6 for now.
Experimental Group class added.
User.groups and Group.users can be used to modify group membership
using the "add" and "delete" methods.

Some opener code was touched as well. It was doing something to try
and "pre-register" URLs with the opener, probably to make sure the
opener associated with the authentication method was used. This doesn't
seem to be necessary as long as we explicitly use the right opener
to begin with. Reason for poking with this to begin with: it generates
extra GET requests, and some of the Group API URLs respond with 404
to GET requests.