Second submit newsletter

https://www.pivotaltracker.com/story/show/82978346
I found there is no change in my first submission. So I resubmit it. 
I have do some modification following your suggestions. I check the initialized variables, and the updates values after calling the method. Because I couldn't call the function explicitly, I just try to set the mode to instant, and do_send to be true, which might send newsletters for me automatically. However, I can only check the last_user_id is exactly the last person's id. I set the CHUNK_SIZE 100, but I cannot prove any difference, if there are more people than 100. I really appreciate your review and help.
looking at the source code, I just realized this does not affect the methods tested in this spec file. We should be able to remove this stub without affecting the results of the tests. The less code we introduce, the better :grinning: