allow for role exclusion

I am including a patch to allow for role exclusion from auto usernaming.

Seemed like best logic was exclusionary rather than inclusion - in other words, by default no roles are selected and ALL users will be affected by AUN (as the module original did). But admin can now set any roles which they don't want to be affected - such as admins (as in my case).

Also, since no roles on registering - AUN will always initially apply.