acme/autocert: add Listener

Now users can do 1-line LetsEncrypt HTTPS servers:

    log.Fatal(http.Serve(autocert.NewListener("example.com"), handler))

Updates golang/go#17053

Change-Id: I13fcd3985ebf6bc97a7524cceeb7641cf1b66b22
Reviewed-on: https://go-review.googlesource.com/39207
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>