Change model __unicode__ method to __str__
Change model __unicode__ method to __str__

* Add python_2_unicode_compatible decorator.
* Make str() / unicode() work in both Python 2 and 3.

Fixes #135

yprez/django-post_office,RafRaf/django-post_office,JostCrow/django-post_office,jrief/django-post_office,ui/django-post_office,ui/django-post_office