Self logging of grades.

See #203.

The `find_student` method on Grade kept returning assignments instead, so I stopped using it in favor of User.find.

Lines 103 and 104 in controllers/grades_controller.rb were changed so that the new Grade object would get the correct properties from Assignment.

Line 22 in controllers/info_controller.rb closes issue #301.

Line 116 in `cache_associations` on Grade and `team_for_course` on User both didn't work as previously written. They work fine now.

Remaining issue now: line 54 on Grade (in `score`) does not work because `raw_score` is `nil`. `assignment_weight` is set properly, however.