Updated the book to reflect Marek Sikora's feedback.
Updated exercise in chapter 6
Merge pull request #3 from canibanoglu/category-url-map

Fixed the category url-view mapping so that Django doesn't raise a 404
Updated the book to include fix by @canibanoglu for the category URL pattern.
Merge branch 'master' of https://github.com/leifos/tango_with_django_book