Fix for #25 for Python 3
Fix for #25 for Python 3

Python 3 uses __bool__ instead of __nonzero__ - following
the  suggestions on
https://docs.djangoproject.com/en/dev/topics/python3
/#boolean-evaluation to fix.

FrankSalad/django-recurrence,Nikola-K/django-recurrence,linux2400/django-recurrence,linux2400/django-recurrence,django-recurrence/django-recurrence,Nikola-K/django-recurrence,FrankSalad/django-recurrence,django-recurrence/django-recurrence