Update assignment model
* Add method for calculating percentage of students that have
  submitted an answer for an assignment
* Add method for returning all answers to an assignment that
  have been marked as accepted
* Add method for calculating percentage of students have an
  accepted answer for an assignment
add comment to assignment model
update staff assignment index page to display progress bar for answer submissions
Merge pull request #71 from cmckee-dev/ryan/assignment_completion

Assignment Answers Indicator