Implement S3 Gateway to third party cloud storage providers. (#3756)

Currently supported backend is Azure Blob Storage.

```
export MINIO_ACCESS_KEY=azureaccountname
export MINIO_SECRET_KEY=azureaccountkey
minio gateway azure
```

cmd/gateway-startup-msg_test.go

@@ -0,0 +1,31 @@
+/*
+ * Minio Cloud Storage, (C) 2016, 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 "testing"
+
+// Test printing Gateway common message.
+func TestPrintGatewayCommonMessage(t *testing.T) {
+	apiEndpoints := []string{"127.0.0.1:9000"}
+	printGatewayCommonMsg(apiEndpoints, "abcd1", "abcd123")
+}
+
+// Test print gateway startup message.
+func TestPrintGatewayStartupMessage(t *testing.T) {
+	apiEndpoints := []string{"127.0.0.1:9000"}
+	printGatewayStartupMessage(apiEndpoints, "abcd1", "abcd123", "azure")
+}