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.
The line isn't called because the line above it says: 

context.stop(self)

But I'm assuming context.stop(self) causes an actor to exit its current scope.