Saves current task information to active file.

            Example format::
                active_task {
                    name "task name";
                    start_time "2012-04-23 15:18:22";
                }

def _save_active_file(self):
        """ Saves current task information to active file.

            Example format::
                active_task {
                    name "task name";
                    start_time "2012-04-23 15:18:22";
                }
            """

        _parser = parser.SettingParser()

        # add name
        _parser.add_option(None, 'name', common.to_utf8(self._name))

        # add start time
        start_time = self._start_time.strftime('%Y-%m-%d %H:%M:%S.%f')
        _parser.add_option(None, 'start_time', start_time)

        # write it to file
        return _parser.write(self._paths['active_file'],
                             self.HEADER_ACTIVE_FILE)