Make it possible to override registration_role when user is created by an admin

As a side effect of #362067: Conflict with Login Tobaggon the roles of a user created by an administrator will now always contain the RR assigned role. The prior method gave full control to the admin/user/user/create form so users created by an admin had exactly the roles selected on the admin form. I have 2 potential patches:

first, http://drupal.org/files/registration_role-ignore_admin.patch
Adds a checkbox to the RR configuration that allows the admin to ignore the addition of a RR specified role when a user is added via the admin user_register form.

second, http://drupal.org/files/registration_role-admin_default.patch
From a UI perspective the first idea didn't make sense after I had already built it, so I came up with this. Grab the RR role and check the box for that role on the admin user_register form by default and always ignore the RR role from the admin user_register form.