Make shop templates extend shop/base.html.

The change allows a site to supply shop-specific base.html, potentially
overriding or extending the global base.html.
Add SHOP_USE_RATINGS (default True), allowing ratings to be disabled.
Added Mike Wakerly to AUTHORS.
Merge
Merge
Remove deprecated ADMIN_MEDIA_PREFIX.
Use Django's timezone support since Django 1.3 is not supported.