encode unknown protocol addresses as strings

[gaboose]

I can add new protocols from outside of this library with a line like

ma.AddProtocol(ma.Protocol{42, -1, "dns", ma.CodeToVarint(42)})

But it fails when I call NewMultiaddr("/dns/google.com"), because it doesn't know how to encode "google.com". What if you encode unknown protocol addresses of variable length as bare strings?

This change makes this library work.

[jbenet]

we'd have to length prefix them, see how i proposed unix recently