Add address family as optional UDPSocket constructor argument

There was an issue reported (#78).

`Celluloid::IO::UDPSocket` wrapper does not accept any arguments to underlying `::UDPSocket`.

`::UDPSocket` has address family set by default to AF_INET. If you need to use IPv6, you need to specify `::Socket::AF_INET6` as an argument to `::UDPSocket` constructor, which is missing if you are using wrapper.