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
