Dossier: Fixed start_date default value

Use a `datetime.date` object for a schema.Date field instead of `datetime.datetime`.
Added `checked_out_by` possibility to the `DocumentBuilder`.
Reworked the Dossier Base tests.
Dossier resolving: Fixed `its_all_closed` check, for nested subtasks.
Merge pull request #189 from 4teamwork/pg_dossier_is_all_closed_fix

Dossier close:  Fix `is all closed ` check.