Added api/assignments/:id/groups
Cleanup main api controller comments
Update SubmissionDownloadsController
Created tests for controller, and also fixed a bug in
submission.rb's get_submission_by_group_and_assignment
MainApiController should render 404's for all routes by default
Simplify GroupsController
Merge pull request #1062 from danielstjules/issue-1002

Issue 1002: Add groups controller, update submission_downloads