Add ability to get_scores() and do some minor submissions API cleanup.

This is what will be called by the LMS when querying the submissions
app for the raw scores of a student in a particular course.

[TIM-232]
I don't know if we were going for irony, but it bugged me to have a grammar error in a prompt partially about grammar.
Merge pull request #127 from edx/ormsbee/score_summaries

Get scores by course_id and student_id