Created ProductListView to filter out inactive products.

The ListView being used to display the product catalogue did not filter out inactive products. This led to some strange behaviour by default, e.g. if a shop had an inactive product it would be visible in the catalogue but when you tried to add it to the cart it would not be added.

I implemented a ProductListView class with an overriden get_queryset method to fix this, and I edited the urls.py to point to it.

I also fixed a small typo in the product.py file while I was there.