Added config file to allow customization of class names and tables.

I have seen a couple posts for people requesting to be able to customize table names and change the default models used.  I added a config file that allows you to do just that.  It "ships" with some handy defaults.  It will allow a developer to also namespace their models.  Just change the model that should be used as the permission and role model and the config will keep the models updated.

Note: The user model used by Entrust is updated using the `auth.php` config file.