Assignment hueristics and more

1) Tracks which assignment was last edited, so that when program is reopened, it will open to that assignment.
2) Makes the course autosave when a cell is edited, so that the professor shouldn't ever have to hit File->Save (though that exists for whoever's precaution). Also added functionality to "Save All".
3) Add Student Menu now exists. Also displays a table of current students, where first and last name can be edited.