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