allow port sharing for UDP sockets, issue #46

exclusive access to an `ADDRESS:PORT` pair is not ideal for multicast use, the proposed fix was provided in the original issue comments - SO_REUSEPORT allows multiple bindings to address and port while SO_REUSEADDR only allows reuse of the address:
http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man2/setsockopt.2.html
http://code.google.com/p/cocoaasyncsocket/issues/detail?id=46