Grader refactor

 * Added back old variable-value-based grader because needed exercises without function definitions.
 * Moved graders to their own objects away from ParsonWidget:
   * ```VariableCheckGrader``` executes code and checks variable values after execution
   * ```UnitTestGrader``` executes unit tests
   * ```LineBasedGrader``` is the original grader based on line order comparison (note: I did not fix the existing bugs with same lines in this grader)