Slightly improve admin interface

Provide missing label for WaitingListEntry, which only becomes a problem in cases where an admin/staff needs to manually update a SUrveyInstance.

Previously the entry dropdown would only contain the values "WaitingListEntry object".

Now, it could contain the email address of the waitinglist object.
Update models.py

re-indent blank line as per contribution guidelines
Merge pull request #4 from airtonix/patch-1

Patch 1