Gradebook class with the following behavior:
>>> book = Gradebook()
>>> book.addGrade('Moe', 50)
>>> book.addGrade('Moe', 90)
>>> book.hasGradesFor('Moe')
True
>>> book.allGradesFor('Moe')
[50, 90]
>>> book.averageGradeFor('Moe')
70.0
>>> book.hasGradesFor('Larry')
False