Allow adding Auditors with the singular noun.

Summary: Currently adding auditors in commit message only works with the plural. Make "Auditor: bla" work.

Test Plan: Unit tests. I don't know if I am supposed to add a test.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: joshuaspence, epriestley

Differential Revision: https://secure.phabricator.com/D10636