Implement Circuit Relay v2 and DCUtR

[dariusc93]

Not sure if this is something that would be desired to be implemented, however implementing these 2 behaviours would go a long way for talking to peers behind a NAT. It would require a ClientTransport from Relay protocol to be apart of the transport and in general is a simple implementation and could be left up to the users of this crate on how they wish to connect to a relay.

EDIT: Looks like this may belong in #5 instead?