Just to be complete: rtc.io has a signalling server at https://github.com/rtc-io/rtc-switchboard . Choice of library is up to you. :-)
https://github.com/rtc-io/rtc-signaller
That said, it wouldn't take much to write an signalling client that followed the basics of the messaging format if you did want to roll your own:
https://github.com/rtc-io/rtc-signaller#signaller-message-ha...