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
I still am not 100% sure how I want to handle socket options that are specific to a single platform. I don't know if scoping it as you did here or making it a no-op on unsupported platforms is better. Either way, this seems fine for me.