Several improvements to the e-mail support here.

* Recipient logic is now improved. We always e-mail to the review request author and the target reviewers, but we also add to this in some cases:
  * Updates to a diff or review request go to every person who has been involved in a discussion on the review request.
  * A reply on a review goes to every person who has been involved in that review's discussion.

* We now use Django's e-mail support instead of our own, now that we can subclass their new EmailMessage object and inject the headers we need. This also gives us the next feature..
* Unit tests! We now unit test e-mail. We specifically check the subject and the To: list.

Ran the unit tests. They worked.