Handle taking registrations on a course with TBD start date

(EDITED 1/27/14 11:48 PST)

At Stanford we sometimes would like to advertise a course publicly before committing to a start date.  This small patch allows us to do that.  If `course.start` is left as default and `course.advertised_start` is not set, then the LMS won't display start date in the tiles, the course about page, or the dashboard.