Add notifications by webhook.

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

cmd/notify-webhook_test.go

@@ -0,0 +1,79 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"path"
+	"testing"
+
+	"github.com/Sirupsen/logrus"
+)
+
+// Custom post handler to handle POST requests.
+type postHandler struct{}
+
+func (p postHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		http.Error(w, fmt.Sprintf("Unexpected method %s", r.Method), http.StatusBadRequest)
+		return
+	}
+	io.Copy(w, r.Body)
+}
+
+// Tests web hook initialization.
+func TestNewWebHookNotify(t *testing.T) {
+	root, err := newTestConfig("us-east-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(root)
+
+	_, err = newWebhookNotify("1")
+	if err == nil {
+		t.Fatal("Unexpected should fail")
+	}
+
+	serverConfig.SetWebhookNotifyByID("10", webhookNotify{Enable: true, Endpoint: "http://www."})
+	_, err = newWebhookNotify("10")
+	if err == nil {
+		t.Fatal("Unexpected should fail with lookupHost")
+	}
+
+	serverConfig.SetWebhookNotifyByID("15", webhookNotify{Enable: true, Endpoint: "http://%"})
+	_, err = newWebhookNotify("15")
+	if err == nil {
+		t.Fatal("Unexpected should fail with invalid URL escape")
+	}
+
+	server := httptest.NewServer(postHandler{})
+	defer server.Close()
+
+	serverConfig.SetWebhookNotifyByID("20", webhookNotify{Enable: true, Endpoint: server.URL})
+	webhook, err := newWebhookNotify("20")
+	if err != nil {
+		t.Fatal("Unexpected shouldn't fail", err)
+	}
+
+	webhook.WithFields(logrus.Fields{
+		"Key":       path.Join("bucket", "object"),
+		"EventType": "s3:ObjectCreated:Put",
+	}).Info()
+}