${cmd_name}: list messages in the specified folder

        ${cmd_usage}
        ${cmd_option_list}
        SINCE can be used with epoch times, for example: 

          md ls -s $(date '+%s')

def do_ls(self, subcmd, opts, folder=""):
        """${cmd_name}: list messages in the specified folder

        ${cmd_usage}
        ${cmd_option_list}
        SINCE can be used with epoch times, for example: 

          md ls -s $(date '+%s')
        """
        client = MdClient(self.maildir, filesystem=self.filesystem)
        client.ls(
            foldername = folder, 
            stream = self.stdout, 
            reverse = getattr(opts, "reverse", False),
            grep = getattr(opts, "grep", None),
            field = getattr(opts, "field", None),
            since = float(getattr(opts, "since", -1))
            )