WIP: API Doc

@dsjen   I did get it in testing:

{
  "interval_start": "2014-12-08T00:00:00Z",
  "interval_end": "2014-12-15T00:00:00Z",
  "activity_type": "posted_forum",
  "count": 1758,
  "course_id": "edX/DemoX/Demo_Course"
}

OK to keep?