Adjusting Group Membership user comparison

By using User#user_key, we are allowing the behavior for implementors
that chose to use a username as the identifier.

@TODO - need to craft the GroupMembership idea. When we iterate on a
Groups#members, we should be providing a proper object that can be
more easily interrogated for its responsibilities.
Merge pull request #538 from ndlib/associating-groups-user-key

Adjusting Group Membership user comparison