api/submit - the backend part for unsubmitting the last submission

This is the backend API endpoint for the future ```exercism unsubmit``` command. This command is part of the fix for #135.

This API endpoint can also be used in the future JavaScript code for deleting the last submission via the web UI.

The following rules are applied:
```
    raise NothingToUnsubmit.new  if submission.nil?
    raise SubmissionHasNits.new  if submission.this_version_has_nits?
    raise SubmissionApproved.new if submission.approved?
    raise SubmissionTooOld.new   if submission.older_than?(TIMEOUT)
```