Open.nat

@Nicolas, I am sending this Pull Request for your evaluation. I would love to discuss with you any issue and answer any question. CanUseUpnp test pass after my modifications.

Here you have a short list of [reasons to use Open.NAT](https://github.com/lontivero/Open.Nat/wiki/Why-Open.NAT%3F) and I hope it is useful for NBitcoin.