apps.core.models.examiner: Added bulkmake_examiner_for_groups.

See #554.
apps.core.models.examiner: Made bulkmake_examiner_for_groups handle

adding multuple examiners to the given groups. Renamed it to
bulkadd_examiners_to_groups and added example better tests and better
docs.
apps.core.models.examiner: Added bulkclear_examiners_from_groups.

See #554.
core.models.examiner: Added randomdistribute_examiners().

See #554.