Add mail-listener atmosphere package.

From https://github.com/BenjaminRH/meteor-mail-listener via
https://atmosphere.meteor.com/package/mail-listener

Not actually installing with meteorite; I'm checking this in as
a standard meteor package.

Change-Id: I6c644615a384556c14b1f19f4deccc1543dd3f6b
Added mail listener!

Updated the mail-listener package to use mail-listener2, and reworked it
to properly wrap callbacks. (https://github.com/chirag04/mail-listener2)

Implemented a listener which posts messages from 'thehunt' in general chat
when HQ sends us mail.  Whoo!

Closes: #91
Change-Id: I2de309979b7a91911537007aabb63594534791fa