Write account configurations in an atomic way

        filename = os.path.join(self._path, '%s.json' % login)

        self.logger.debug('Writing configuration %s: %s', filename, conf)

        if os.path.isfile(filename) ^ create:

            json.dump(conf, open(filename, 'w'))

            with open(filename + '.tmp', 'w') as ftmp:

                json.dump(conf, ftmp)

            os.rename(filename + '.tmp', filename)

        else:

            raise CCConf.UnknownAccount('%s is not a file' % filename)