Adds sample student communications (email msgs)

@mmacfarlane , @WatsonEmily , please review and provide many opinions on what to add/delete/change. An html version is available here: http://draft-student-communications.readthedocs.org/en/latest/releasing_course/course_launching.html

@mhoeber , @srpearce please copy edit and proofread.
Follow instructions in the formatting cheat sheet to change the link text to avoid awkward capitalizations, or change to "This chapter also includes an "Example Messages to Students" section that you can use when you write your own email messages."