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.
ProtocolFamily and StandardProtocolFamily is Java7: http://docs.oracle.com/javase/7/docs/api/java/net/ProtocolFamily.html

This'd mean that Akka wouldn't work with Java6.