v0.5 - Fixed relationships, started using GradeLevel, et.al.

1) Bumped version to v0.5.
2) Fixed (all?) of the relationships.
3) Updated to use GradeLevel instead of GradeYear.
4) Added Student.gpa as part of replacing GradeYear
   by GradeLevel.
5) Added override_grade_id to StudentClass to document
   that student 'student_id' is allowed to take a
   class not in their GradeLevel.
6) Fixed POD comments in Grade.
7) Fixed POD comments in Student.