Create context-named connections

This way, it becomes possible to add distinct ldap connectors
to the same pyramid app, and reference them by adding a context=...
discriminator to the api calls:

    ldap_setup()
    ldap_set_login_query()
    ldap_set_groups_query()
    get_ldap_connector()

and to the Connector's __init__ method.

Feature comes complete with unit tests...
Change authenticated_user's representation

to help groupfinder discriminate between configured backend contexts
No useless variables...
Adapt tests to the new representation.