use djangos default permission_denied view instead of the raw HTTPResponseForbidden

In the section "Using page request processors" use djangos default permision_denied view instead of the HttpResponseForbidden. This will automatically pickup the 403.html template or return the string 403 Forbidden.
use PermissionDenied exception in the page request processor example

- In the page request processor example raise a PermisionDenied exception instead of calling the permission_denied view directly.
- Updated the docs for the request processor to mention that if an exception is raised it will be handled like all exceptions are handled in django views.
Merge pull request #473 from adsworth/patch-1

in request_processor example raise PermissionDenied instead of returning HttpResponseForbidden