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/main.go

@@ -98,6 +98,7 @@ func newApp() *cli.App {
 	registerCommand(serverCmd)
 	registerCommand(versionCmd)
 	registerCommand(updateCmd)
+	registerCommand(gatewayCmd)
 
 	// Set up app.
 	cli.HelpFlag = cli.BoolFlag{