Created example of support for binary key formats.

There are various performance reasons to use binary key formats (and of course good debugability reasons not to). Redis certainly supports this. 

Near as I can tell, support for this is already present in the backend of rediscala. It's merely the API that doesn't expose it, but please correct me if I'm wrong if I missed something. 

This pull request exposes the binary API in the `setB` and `existsB` methods. I'd be happy to port more methods over if this seems like a useful API.