Return completed submissions to peer-grader if no pending ones
Address review comments and conditionalize peer-grading finished subs
changes according to @VikParuchuri comments

* add submissions_graded > 0 back in
* order_by('-date_modified')
* preferred_grader_type = PE
Merge pull request #140 from edx/jbau/return-finished-submissions-to-grade-when-all-finished

Return completed submissions to peer-grader if no pending ones