refactor for transport changes

Also, make the libp2p constructor fully useful. There should now be no need to
manually construct a swarm/host.

examples/echo/main.go

@@ -49,7 +49,7 @@ func makeBasicHost(listenPort int, secio bool, randseed int64) (host.Host, error
 	}
 
 	if !secio {
-		opts = append(opts, libp2p.NoEncryption())
+		opts = append(opts, libp2p.NoSecurity)
 	}
 
 	basicHost, err := libp2p.New(context.Background(), opts...)