Add comment editing and deleting functions to canvas.submission.Submi…

[laitingsheng]

Proposed Changes

This PR addresses issue #490 by adding two additional methods to canvasapi.submission.Submission. To complete the testing, I've also added the optional "submission_comments" field to the original submission fixture. Technically, this will only be returned if you add include=["submission_comments"] to GET /api/v1/courses/:course_id/assignments/:assignment_id/submissions/:user_id, which is corresponding to canvasapi.assignment.Assignment.get_submission. That's why I updated the __init__ method for canvasapi.submission.Submission as well.

Fixes #490.