Added list_submissions api command

* Added _get_submission
* Added _submission_to_str and _test_result_to_str
Only list submissions with test_results.
list_submission had test results backwards.
Added change_submission_grade call. Closes #303.
Added modify_user_deadline command. Closes #302.

* Added personal_deadline MapField to User model
* Modified browse_assignments and resumbit views to consider personal deadline.
Merge commit '4e3494c212d7de079d42483a10d35e9ef7a1f1db' into v0.2stable