lib/pmp: Add NAT-PMP support (ref #698)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2968

vendor/github.com/AudriusButkevicius/go-nat-pmp/README.md

@@ -0,0 +1,48 @@
+go-nat-pmp
+==========
+
+A Go language client for the NAT-PMP internet protocol for port mapping and discovering the external
+IP address of a firewall.
+
+NAT-PMP is supported by Apple brand routers and open source routers like Tomato and DD-WRT.
+
+See http://tools.ietf.org/html/draft-cheshire-nat-pmp-03
+
+Get the package
+---------------
+
+    go get -u github.com/jackpal/go-nat-pmp
+
+Usage
+-----
+
+    import natpmp "github.com/jackpal/go-nat-pmp"
+
+    client := natpmp.NewClient(gatewayIP)
+    response, err := client.GetExternalAddress()
+    if err != nil {
+        return
+    }
+    print("External IP address:", response.ExternalIPAddress)
+
+Notes
+-----
+
+There doesn't seem to be an easy way to programmatically determine the address of the default gateway.
+(Linux and OSX have a "routes" kernel API that can be examined to get this information, but there is
+no Go package for getting this information.)
+
+Clients
+-------
+
+This library is used in the Taipei Torrent BitTorrent client http://github.com/jackpal/Taipei-Torrent
+
+Complete documentation
+----------------------
+
+    http://godoc.org/github.com/jackpal/go-nat-pmp
+
+License
+-------
+
+This project is licensed under the Apache License 2.0.