Methods for reading reviewers from edges in differential

Summary: Add `getReviewerStatus` to get an array of `DifferentialReviewer` objects. The method `needReviewerStatus` in `DifferentialRevisionQuery` loads the edges into the revisions loaded.

Test Plan: Added `->needReviewerStatus(true)` to `DifferentialRevisionSearchEngine` and checked through logging that the data was being loaded correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6450