Refactor overall_rating and user_rating tag as built in Django simple_tag
Refactor overall_rating and user_rating tag as built in Django simple_tag

assignment-tags has been deprecated since version 1.9:
simple_tag can now store results in a template variable and should be used instead.
https://docs.djangoproject.com/en/1.9/howto/custom-template-tags/#assignment-tags

ratings tags as also been replace with simple_tag

Closes #30 #31

arthur-wsw/pinax-ratings,arthur-wsw/pinax-ratings,arthur-wsw/pinax-ratings,pinax/pinax-ratings,pinax/pinax-ratings,pinax/pinax-ratings