- Changed the names of CourseSummary, ExerciseRoundSummary and CategorySummary by prefixing them all with "User" (see the previous commit message for more information).
- Changed the way that UserExerciseSummary takes the kwargs submission_count, best_submission and generate as I think they should be invisible to public callers as they are meant to be used in UserCourseSummary only really.
- Documented UserExerciseSummary.