Add properties to differential.getdiff

Summary: Add 'addLines' and 'delLines' properties to differential.getdiff return
dictionary. These properties are aggregated from the changesets.

Test Plan: Issue a differential.getdiff query via conduit and verify that
'addLines' and 'removeLines' properties are included and accurate

Reviewers: epriestley

Reviewed By: epriestley

CC: epriestley, aran, jonathanhester

Differential Revision: 1209