Add differential.query conduit method

Summary:
Created a differential.query conduit method that is built on top of
DifferentialRevisionQuery. I also added support for querying by author, ccs, and
reviewers to DifferentialRevisionQuery, so feature parity can be brought up to
match differential.find and its backing class DifferentialRevisionListData.

Test Plan:
Tried a few calls to the conduit call using the web interface, and got back
reasonable looking data.

Reviewers: epriestley, jungejason, btrahan

Reviewed By: epriestley

CC: aran, nh, epriestley

Differential Revision: 1158