Exercise completion status for the course page

Exercise completion status of logged in user is shown on the course page as a green/red bar, see: http://www.cs.helsinki.fi/u/mluukkai/exercise_status.png

Course controller sets a value to exercise.completion_status_for_current_user based on the info generated by code imlemented in lib/ExerciseStatusGenerator 

The location of green/red bar figure is now hardcoded since I was not sure where it should be put in the rails directory structure