Add notifications by webhook.

Add a new config entry moving to version 13.
```
		"webhook": {
			"1": {
				"enable": true,
				"address": "http://requestb.in/1i9al7m1"
			}
		}
```

cmd/globals.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"crypto/x509"
 	"os"
+	"runtime"
 	"strings"
 	"time"
 
@@ -36,7 +37,7 @@ const (
 
 // minio configuration related constants.
 const (
-	globalMinioConfigVersion      = "12"
+	globalMinioConfigVersion      = "13"
 	globalMinioConfigDir          = ".minio"
 	globalMinioCertsDir           = "certs"
 	globalMinioCertsCADir         = "CAs"
@@ -96,6 +97,9 @@ var (
 	// List of admin peers.
 	globalAdminPeers = adminPeers{}
 
+	// Minio server user agent string.
+	globalServerUserAgent = "Minio/" + ReleaseTag + " (" + runtime.GOOS + "; " + runtime.GOARCH + ")"
+
 	// Add new variable global values here.
 )