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