Unicode method on model should return unicode

The example of a \__unicode\__ model method should return a unicode value
instead of a string.

This helps us avoid errors like [these](http://twigstechtips.blogspot.com/2011/10/django-how-to-fix-annoying.html):

    DjangoUnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 52: ordinal not in range(128)