code: public Properties toProperties()
    {
      Properties props = new Properties();
      if(passwordHash != null)
      {
        props.put("password", passwordHash);
      }
      if(groups != null && !groups.isEmpty())
      {
        props.put("groups", Joiner.on(',').join(groups));
      }
      if(permissions != null && !permissions.isEmpty())
      {
        props.put("permissions", Joiner.on(',').join(permissions));
      }
      if(expires != null)
      {
        props.put("expires", ISODateTimeFormat.date().print(expires));
      }
      return props;
    }
docs: Constructs a represention that is equal to the content of an ANNIS
  user file.
  @return