Removed hardcoded url's in polls view 'index'

By letting the url dispatcher handle the url's, we can easily relocate
url's in the app's URLconf module.

Customization and flexibility does not get any better than this.
Namespacing URL names to avoid view name conflicts

Since the same view name could occur in different apps in the project,
Django allows us to specify a namespace when setting up the url patterns
table in the root URLconf module.