Implements OGDSService class which provides methods to query users from the ogds database.

At a later date the service should also provided mehtods to query clients, organisational and administrative units.
Reworked Client tests.
Add `assigned_users` getter for a `Client`.
Added methods to query the clients to the `ogds_service`.
Split up `Client` model in to separate wrapper classes `AdminUnit` and `OrgUnit`.

This adjustment is part of the ogds client concept refactoring,
with the goal to cleary spliting up the business locig unit and
the installation.
Implements `AdminUnit` as a separate table instead of a client wrapper class.
Use proper super-calls instead of ... uhm.
Add helper method to detect if multiple admin units are present.
Rework `_query_admin_units` and use it consistent.

Made `enabled_only` as keyword arguments.
Define client query and ordering on title per default.
Define user query in a separate method.
Merge pull request #6 from 4teamwork/pg_user_model_extraction

Implements OGDSService class which provides methods to query from OGDS database