Implement ScoringProtocol

Resolves #68 . `ScoringProtocol` is a light wrapper around the `GradingProtocol`'s grading mechanism -- only the output is different. I've tested it with the command-line (scoring can be activated with the `--score` flag) and it seems to work. Here's some sample output:

    Point breakdown
    ===============
    q1: 3.0/3
    q2: 0.5/1

    Total score:
    3.5

https://github.com/Cal-CS-61A-Staff/ok/blob/client-scoring/client/protocols/grading.py#L58 onwards. It looks like the main difference is the scores() dictionary in the scoring protocol versus the lack thereof in the GradingProtocol.