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/api-errors.go

@@ -692,6 +692,8 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 		apiErr = ErrEntityTooLarge
 	case ObjectTooSmall:
 		apiErr = ErrEntityTooSmall
+	case NotImplemented:
+		apiErr = ErrNotImplemented
 	default:
 		apiErr = ErrInternalError
 	}