Fix string representation of answer models

Hi, thanks for domande, it's a nice and clean library!
While hacking around I noticed that the admin interface couldn't handle displaying answers; the issue was that their __unicode__ method returned the question model instead of its string representation :)

By the way, the pypy repository still has an old version with the comma missing from the requirements. People would probably be happy if there was a newer version available!

Thanks again,
best,
Pascal