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
For what it's worth, I figured since Multicast would apply UDP sockets only, really, that it'd make sense to separate it from Socket itself (but still "be" a socket, since it otherwise looks and smells like one).