fix for gmail smarthosts

allow the servername used for authentication to be different from the
smarthost_server variable. In case of gmail you need smtp.gmail.com as the
smarthost, but in passwd.client you need *.google.com...
Merge pull request #2 from jkraemer/gmail_smarthost_fix

Merged fix for gmail smarthosts. New default attribute needs to be added as well (and documented).