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_test.go

@@ -103,6 +103,10 @@ func TestAPIErrCode(t *testing.T) {
 			StorageFull{},
 			ErrStorageFull,
 		},
+		{
+			NotImplemented{},
+			ErrNotImplemented,
+		},
 		{
 			errSignatureMismatch,
 			ErrSignatureDoesNotMatch,