docs.core.models: Added AssignmentGroupManager and AssignmentGroupQuerySet.
apps.core.models.assignment_group: Added group_by_tags() to manager and queryset.
devilry_developer.testhelpers.corebuilder: Added tag support to AssignmentGroupBuilder.
apps.core.models.assignment_group: Removed group_by_tags() from manager.

It never makes sense to fetch ALL groups and group them by tags, so
we only need to have it on the queryset.

Documented group_by_tags() better.
devilry_developer.testhelpers.corebuilder: Use bulk_add_candidates_to_groups.
apps.core.models.assignment_group: Simplified __unicode__.
apps.core.models.candidate: Added only_candidate_in_group, and updated

CandidateManager.bulk_add_candidates_to_groups including tests.
See #574.
core.models.candidate: Docs.