Migrate unlocking feature from old autograder.

This pull requests implements the unlocking functionality from the old autograder. The `Protocol` is not yet implemented (I'll do this in a separate pull request).

Some code for the grading protocol has also been rewritten to be more consistent (including the introduction of an `OkConsole` abstract class).

This resolves #53 .
To avoid treating concept questions the same way, can't we just have a "is_graded" flag which is set if and only if it's going to be graded in grading.py.