acme/autocert: support http-01 challenge type

The Manager now loops through known challenge types,
trying to fulfill one at a time until it succeeds or runs out
of supported challenges.

The provisioning of "http-01" challenges can be done
using the new Manager.HTTPHandler method.
It requires listening on unencrypted port 80.

Fixes golang/go#21890

Change-Id: I55de9501f0069a9f460fedd8b5b0a09b94f9ef05
Reviewed-on: https://go-review.googlesource.com/87201
Run-TryBot: Alex Vaghin <ddos@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/autocert/example_test.go

@@ -22,11 +22,12 @@ func ExampleNewListener() {
 }
 
 func ExampleManager() {
-	m := autocert.Manager{
+	m := &autocert.Manager{
 		Cache:      autocert.DirCache("secret-dir"),
 		Prompt:     autocert.AcceptTOS,
 		HostPolicy: autocert.HostWhitelist("example.org"),
 	}
+	go http.ListenAndServe(":http", m.HTTPHandler(nil))
 	s := &http.Server{
 		Addr:      ":https",
 		TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},