Added cjdns

cjdns - Caleb James DeLisle's Network Suite

A tool for building physical and soft meshnets through virtual IPv6
interfaces.

KNOWN ISSUE:
This formula only worked with superenv off (i.e. `brew install --env=std cjdns`). It's quite a hassle, and there's no documentation on how to force that behavior in homebrew, so the most I could do was include a warning about it.