Add Atom export to Phame

Summary:
There's no link to this yet, I'll add it to some skin.

Fixes T2272.

Test Plan: http://validator.w3.org/appc/

Reviewers: btrahan, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2272

Differential Revision: https://secure.phabricator.com/D4366