R1:b4d9a8d54712

Add a "before" parameter to feed.query

Summary: See IRC. We already have "after", add the corresponding "before". This makes polling for updates much easier.

Test Plan: Ran queries with "before" and "after".

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6903