Add path support to differential.query

Summary:
Allows clients to select open differential revisions that
affect a set of paths.

Test Plan:
http://phabricator.mkedia.dev2975.facebook.com/conduit/method/differential.query/ with

paths: tabloid"
status: "status-open"

(internal facebook repo, obvi)

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: vrana, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2788