Add option to redirect succesful logins based upon the requested URI

With this patch, an option is added to redirect succesful Raven logins to the requested uri. This enables the following:

* admin sets the site 403 page to raven/login
* user clicks on a link to a page which they are not authorized to view
* user is automtically redirected to the Raven login page
* upon succesful authentication, they are redirected back to the requested page

The user will then either see the page, if authorized, or Drupal will return a generic "not authorized" message if not.