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/natpmp_test.go

@@ -0,0 +1,13 @@
+package natpmp
+
+import (
+	"testing"
+)
+
+func TestNatPMP(t *testing.T) {
+	client, err := NewClientForDefaultGateway()
+	if err != nil {
+		t.Errorf("NewClientForDefaultGateway() = %v,%v", client, err)
+		return
+	}
+}