Support 'authenticated' as role name for authenticated user role

Drupal 8 now throws an exception when trying to assign this role, I have a lot of 'authenticated' in my tests as that is the machine name and other role machine names are supported too, so doing this is consistent I think.

Also lower cased it, because the actual role name is 'Authenticated user'.

Please also backport this to 1.0, as I'm still using that.
Merge pull request #124 from Berdir/patch-1

Support 'authenticated' as role name for authenticated user role