Add orderBy behavior

[esprehn]

firebase-admin-mock was deprecated in favor of this library, but this one doesn't support orderBy while firebase-admin-mock did. That's a really critical feature that prevents us from using this library.

https://github.com/sebasgarcep/firebase-admin-mock/blob/42bd3d862e28004ae97a7eed35b622843ccde05a/lib/database-accesors.js#L81

It's unfortunate that project was immediately archived without having feature parity over here yet. Hopefully firebase-mock can get the same features!

@sebasgarcep