Add course author notes for 7/16 release

@mhoeber @lamagnifica Please review. Draft at http://draft-release-notes-7-16.readthedocs.org/en/latest/07-16-2014.html.

I didn't include the following; let me know if I should:

- https://github.com/edx/edx-platform/pull/4095 (creates an Open edX footer)

- https://github.com/edx/edx-platform/pull/4172 (e-commerce enhancements - I don't think these are externally visible yet)

- https://github.com/edx/edx-platform/pull/4353 and https://github.com/edx/edx-platform/pull/4368 (these two are specific to annotations, which I think only Harvard uses and we don't want to publicize)
@mhoeber , should we include info about the count of active students in the last week in these release notes? I was too late to get them in last week.
Something along the lines of, "The Instructor Dashboard Analytics tab now includes the number of
students who interacted with the course by visiting pages, playing videos, contributing to discussions, submitting answers to problems, or completing other course activites. The active student count is updated each week."