Add a script to close all open audits in a repository

Summary: If you import a repository you may trigger a large number of irrelevant audits. Provide a tool to nuke them.

Test Plan: Ran "audit.php Q" (does not exist), "audit.php P" (phabricator) from various repository states.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904, T940

Differential Revision: https://secure.phabricator.com/D1791