First whack at UDP send/receive sockets for Mio

Add initial support to MIO for UDP sockets, including the ability to
join multicast groups and listen. Still need to add support for
recvfrom/sendto, but works just fine for connected UDP sockets.

Depends on nix-rust having pull request https://github.com/carllerche/nix-rust/pull/12
Are there any other uses for these two traits besides for UDP sockets? It seems like it will probably be better to keep these traits mio::net::udp unless there are other use cases.