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
Good question -- the rationale I had for splitting these out was actually related to UNIX domain sockets. When you have multiple potential clients to a single "listening" UNIX domain socket, you need to use recvfrom/sendto to make sure your message gets to the right destination. Thus, you could treat AF_UNIX sockets like AF_INET[6], SOCK_DGRAM sockets. This also helps for AF_RAW sockets and such, but I'd be surprised if anyone went _that_ far. My thought was that there could be applications that are exclusively directed datagram oriented, so an AF_UNIX, and UDP socket could implement the same traits.