added protocol family argument to Bind message

added new socket option to join a multicast group

This commit provides the means to specify the protocol family (e.g., IPv4 or IPv6) when an actor attempts to bind to a socket using IO(Udp). On systems that are dual stacked, the current implementation will prefer IPv6; preventing a socket from joining an IPv4 multicast group.
Sadly this is not the case, let's see what the documentation says:

"abstract def
stop(actor: ActorRef): Unit
Stop the actor pointed to by the given akka.actor.ActorRef; this is an asynchronous operation, i.e. involves a message send."

Since it is an async operation that involves a message send, it won't "exit it's current scope" (which it could only do by throwing an exception anyway, which could be caught by user code).