Add examples using the bootstrap methods.

examples/README.md

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+# Example
+
+# Client & Server requests
+
+On this example we are going to see the Certificate Authority running, as well
+as a simple Server using TLS and a simple client doing TLS requests to the
+server.
+
+The examples directory already contains a sample pki configuration with the
+password `password` hardcoded, but you can create your own using `step ca init`.
+
+First we will start the certificate authority:
+```
+certificates $ bin/step-ca examples/pki/config/ca.json
+2018/11/02 18:29:25 Serving HTTPS on :9000 ...
+```
+
+We will start the server and we will type `password` when step asks for the
+provisioner password:
+```
+certificates $ export STEPPATH=examples/pki 
+certificates $ export STEP_CA_URL=https://localhost:9000
+certificates $ go run examples/server.go $(step ca new-token localhost))
+✔ Key ID: DmAtZt2EhmZr_iTJJ387fr4Md2NbzMXGdXQNW1UWPXk (mariano@smallstep.com)
+Please enter the password to decrypt the provisioner key:
+Listening on :8443 ...
+```
+
+We try that using cURL with the system certificates it will return an error:
+```
+certificates $ curl https://localhost:8443
+curl: (60) SSL certificate problem: unable to get local issuer certificate
+More details here: https://curl.haxx.se/docs/sslcerts.html
+
+curl performs SSL certificate verification by default, using a "bundle"
+ of Certificate Authority (CA) public keys (CA certs). If the default
+ bundle file isn't adequate, you can specify an alternate file
+ using the --cacert option.
+If this HTTPS server uses a certificate signed by a CA represented in
+ the bundle, the certificate verification probably failed due to a
+ problem with the certificate (it might be expired, or the name might
+ not match the domain name in the URL).
+If you'd like to turn off curl's verification of the certificate, use
+ the -k (or --insecure) option.
+HTTPS-proxy has similar options --proxy-cacert and --proxy-insecure.
+```
+
+But if we use the root certificate it will properly work:
+```
+certificates $ curl --cacert examples/pki/secrets/root_ca.crt https://localhost:8443
+Hello nobody at 2018-11-03 01:49:25.66912 +0000 UTC!!!
+```
+
+Notice that in the response we see `nobody`, this is because the server didn't
+detected a TLS client configuration.
+
+But if we the client with the certificate name Mike we'll see:
+```
+certificates $ export STEPPATH=examples/pki 
+certificates $ export STEP_CA_URL=https://localhost:9000
+certificates $ go run examples/client.go $(step ca new-token Mike)
+✔ Key ID: DmAtZt2EhmZr_iTJJ387fr4Md2NbzMXGdXQNW1UWPXk (mariano@smallstep.com)
+Please enter the password to decrypt the provisioner key:
+Server responded: Hello Mike at 2018-11-03 01:52:52.678215 +0000 UTC!!!
+Server responded: Hello Mike at 2018-11-03 01:52:53.681563 +0000 UTC!!!
+Server responded: Hello Mike at 2018-11-03 01:52:54.682787 +0000 UTC!!!
+...
+```

examples/client.go

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+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"time"
+
+	"github.com/smallstep/certificates/ca"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <token>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	token := os.Args[1]
+
+	client, err := ca.BootstrapClient(token)
+	if err != nil {
+		panic(err)
+	}
+
+	for {
+		resp, err := client.Get("https://localhost:8443")
+		if err != nil {
+			panic(err)
+		}
+		b, err := ioutil.ReadAll(resp.Body)
+		resp.Body.Close()
+		if err != nil {
+			panic(err)
+		}
+
+		fmt.Printf("Server responded: %s\n", b)
+		time.Sleep(1 * time.Second)
+	}
+}

examples/server.go

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+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"time"
+
+	"github.com/smallstep/certificates/ca"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <token>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	token := os.Args[1]
+
+	srv, err := ca.BootstrapServer(":8443", token, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		name := "nobody"
+		if r.TLS != nil && len(r.TLS.PeerCertificates) > 0 {
+			name = r.TLS.PeerCertificates[0].Subject.CommonName
+		}
+		w.Write([]byte(fmt.Sprintf("Hello %s at %s!!!", name, time.Now().UTC())))
+	}))
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println("Listening on :8443 ...")
+	if err := srv.ListenAndServeTLS("", ""); err != nil {
+		panic(err)
+	}
+}