Include explicit "Auditors:" in initial Diffusion email

Summary:
See https://groups.google.com/forum/?hl=en_US&fromgroups#!topic/phabricator-dev/ZonbTN-0TGw

When a commit's author explicitly specifies users as "Auditors", send them an email about the commit.

Test Plan: Made a commit with "Auditors: turtle", ran "reparse.php --herald" on it, verified user "turtle" got included in the intial email.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, MarkChuCarroll

Differential Revision: https://secure.phabricator.com/D3032