devilry_subjectadmin...AssignmentController: Added _isElectronic() method.
apps.administrator...AddDeliveriesMixin: Error handling in addNonElectronicDelivery.

See #407.
versions: Force specific buildout version.
devilry_developer: Added a test settings module.
utils.passed_in_previous_period: Automove latest deadline if in the past.

This solves 2 problems:
- Hard deadlines does not raise exception.
- The "fake" deliveries on soft deadlines is not marked with "delivered
  too late".

Fixes #407.