Add support for working within Network Address Translation (NAT) environments.

This adds a CLI flag `--nat-method` with two options,  `NONE` (the default and 
current behavior) and `UPNP.`  `--nat-method` will affect what IP address and 
ports are advertised via the P2P discovery and other APIs that report the 
address and port for P2P actions.

`UPNP` is intended for a typical home or small office environment where a 
wireless router or modem provides NAT isolation.