Added newsletter spec

https://www.pivotaltracker.com/story/show/82978346
Wrote rspec tests to cover the remaining parts of the newsletter.rb model file.
We should stick to using `expect` over `should`, since the `should` syntax is now deprecated in RSpec 3. There is no real difference anyway!

Asserting that the class of the variable is a `Time` doesn’t tell me much, would it be a better test if we can assert that it was sent at a specific time?