Neater override hooks and more docs for DjangoModelPermissions.

Refs #702.
Update release notes.
Serializers support bulk create.
Failing test to show problem of #720
Fixed #720
Merge pull request #722 from Bouke/patch-1

PrimaryKeyRelatedField with OneToOneField serializes wrong object's id
Added @bouke for bug fix #722. Thanks!
Update release notes
Merge master