Returns url for cdn.urbanterror.info to pass to _not_wget().

        http://cdn.urbanterror.info/urt/<major_ver_without_.>/<release_num>-<magic_number>/q3ut4/<filename>

def _get_url(self, filename):
        """
        Returns url for cdn.urbanterror.info to pass to _not_wget().

        http://cdn.urbanterror.info/urt/<major_ver_without_.>/<release_num>-<magic_number>/q3ut4/<filename>
        """
        return self.cdn_url.format(self.mver, self.relnum, filename)