models: Implement __unicode__ in models instead of __str__

models: Implement __unicode__ in models instead of __str__

According to the Django documentation at [1] it is recommended to
implement __unicode__ and not __str__.  Django's model base class
provides a __str__ method that will use the __unicode__ method and
convert to utf-8.  Also, every text value returned from the DB through
the model is unicode.

It is now possible to edit Patches and Persons that have values with
non-ASCII characters through the admin interface.

[1] http://docs.djangoproject.com/en/1.2/ref/unicode/

Signed-off-by: Dirk Wallenstein <halsmit@t-online.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>