Draft of release notes for 10-7

@lamagnifica @mhoeber @explorerleslie Please review. (Leslie - in particular, can you review the bullets that have to do with discussions?) Built version at http://draft-107-release-notes.readthedocs.org/en/latest/10-07-2014.html.
"subtopic" has not, until now, been used in the discussions.rst or discussions_students.rst files in the B&R guide. if you're using it in the new student-facing doc, then yes, its use here will be consistent with that document. I'd still suggest changing it though.