Remove null and resque messengers

For the purposes of this fork, they aren't desired.
Restructure Messenger and Cheetah classes

* Rename Cheetah::Cheetah to Cheetah::Mailer
* Get rid of Cheetah::Message
* Reorganize Cheetah::Messenger and its specs to soley rely on stubs on
  network requests.
* Temporarily removes email whitelisting.