Added newsletter spec

https://www.pivotaltracker.com/story/show/82978346
Wrote rspec tests to cover the remaining parts of the newsletter.rb model file.
Thanks for taking the time to write these specs, looking good so far (don’t mind my obsessive nitpicking)! A few questions spring to mind when I read the tests. Do these tests (when `Newsletter::SEND_AS == :instant`) belong under the describe block “does not instantly send in scheduler_job mode”? What if the `CHUNK_SIZE` is less than the number of members registered to receive mailings? What if `do_send` was `false`? The tests in this file should be answering these questions.