ProtoBuf.js

#30

While testing I discovered a new enum type in `spotify.metadata.proto.Restriction.Catalogue`.
```
enum Catalogue {
  AD = 0;
  SUBSCRIPTION = 1;
  UNKNOWN = 3;
}
```

I also had to change `spotify.mercury.proto.MercuryReply.statusCode`'s type to `string` as `sint32` has the wrong [wire type](https://developers.google.com/protocol-buffers/docs/encoding#structure). When receiving metadata, `statusCode` is the type of metadata as a `string`.