Pub/Sub support.

A basic Publish/Subscribe support. 
- Actors can be registered as listeners with a client. Support for Akka Streams or scalaz-streams can be added in the future.
- Every listener receives messages from all channels the client is subscribed to.
- Unsubscription affects all listeners registered with the client.
- Payload of the received messages is not deserialized and passed as ByteString wrapped with the Message object to the listener. The reason for this limitation is that I'm not sure, what he best way to register and select the proper deserializer would be. Anyway, it is straightforward to implement the deserialization logic as part of the listener's receive method.

I've implemented this functionality because i would like to use high availability (sentinel) capabilities in my current project and Pub/Sub could be very useful for implementing it. If you are interested in adding the sentinel support to the driver I would be happy to contribute it.