Test coverage for cando.gradebook

cando/      covered 65% (1022 of 2940 uncovered)
    gradebook/      covered 37% (694 of 1108 uncovered)
        browser/      covered 18% (686 of 844 uncovered)
        gradebook.py      covered 95% (8 of 184 uncovered)
        __init__.py      covered 100% (0 of 0 uncovered)
        interfaces.py      covered 100% (0 of 80 uncovered)