NAT traversal stuff

[jbenet]

Hey @fd -- in looking across the net for go implementations of nat traversal I landed here. Looks like you're looking at this too right now. Your abstraction should perhaps be its own package. We'd probably use it if so.

[jbenet]

And, note that tons of homes/offices have two layers of nats: ISP router box + internal wireless router. So will want to try and discover / port map both layers.

[fd]

@jbenet Sure I'll extract it today. I haven't done anything for the double/triple NAT scenarios yet.

[fd]

@jbenet I've moved the subtree to here. I've looked into the double NAT scenario and AFAIK neither UPnP or NAT-PMP support double nat port mapping.