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.
strike "and TAs"
...to assign students to different cohorts in class discussions. --> ...to divide students into cohort groups using either distinctive group characteristics or automated, random assignment.