Corrections to guides for 4 Sep release

Items repaired:
- Pin_discussion.png file in wring directory
- Discussion_filters.png image in discussion_students unreadable,
replaced
- updated  release date in Research guide change log
- updated  release date in Course Author’s guide change log
- updated  “History” dates for fields added to research guide
Merge pull request #5089 from edx/ahodges/doc/repairs20140904

Corrections to guides for 4 Sep release