Fixed the category url-view mapping so that Django doesn't raise a 404

The current url mapping for the category() view is
`url(r'^category/(?P<category_name_url>\w+)$', views.category, name='category'),)`. The problem with this mapping is that this will raise a 404 if the user goes to, say, `/rango/category/Django/` but will work as expected if she goes to `/rango/category/Django`. 
This patch fixes that problem so that both versions work.