feat(csrf): Adds a setting for the CSRF header name.

This is useful for integrating with AngularJS.

AngularJS uses a default cookie and header name to magically _just_ work. It would be nice to set the CSRF cookie and header name in my Django app and not have to worry about setting the CSRF values in my AngularJS code.

Details on the values here:
http://docs.angularjs.org/api/ng.$http  § *Cross Site Request Forgery (XSRF) Protection*