Auth: introduce pattern engine for creating usernames, userhomes

In lieu of Issue #3, the login name entered by the user is now split up
in parts for the workgroup, the username and the domain. These three
parts can be used in patterns to generate the usernames passed to Samba
and LDAP.
Config: fix typo in config file comments
README.md: update with new config settings
Browser plugin: if anonymous_only, don't show logout link