Add userData to handshake

[pfrazee]

Maf mentioned the potential to include userData in the handshake. I'd personally like to use that to declare the intended protocol so that I don't have to go sniffing packets.

[lanmower]

Me likey

[OzymandiasTheGreat]

I've been hacking on the DHT and this seems simple enough to do.
One question tho @mafintosh where should userData be set?
I think it'd be a good idea to set it on the server constructor and connect() call,
that way we can have e.g. custom firewall logic based on userData.
If you clarify this, I'll make a PR.