Added get_top_submissions to submissions API

Added a new API to the submissions module which gets the top
submissions for the assessment
1) The `submissions` API really shouldn't make assumptions about the content of the submission.  It should be the caller's responsibility to retrieve keys from the submission content.

2) You can use `SubmissionSerializer` to do the JSON decoding for you.
```
answer = SubmissionSerializer(score.submission).data['answer']
```