Created the ''docs'' directory to hold the database structure
documentation (made using 'SQL Power Architect'). Note we don't want to
upload it to GAE, so we included it in the skipped files in 'app.yaml'.
For now the database structure docs only list the service tables; from
the '''Auth''' module only the ''auth_user'' table is included, since
the others are not referenced anywhere else in our system.
Added '''auth_user_services''' table to link the services that can be
offered or used by a user (a given service can be offered and used by
many users).