These commits add preliminary support for the TCP/UDP API of mio, built on top
of IOCP using some raw Rust bindings plus some networking extensions as the
foundational support. This support is definitely still experimental as there are
likely to be a number of bugs and kinks to work out.

I haven't yet done much benchmarking as there are still a number of places I
would like to improve the implementation in terms of performance. I've also been
focusing on getting "hello world" and the in-tree tests working ASAP to start
getting some broader usage and feedback. High level docs are available in the
src/sys/windows/mod.rs file and the TCP/UDP implementations are quite similar in
terms of how they're implemented.

Not many new tests were added, but all tests (other than those using unix
sockets) are passing on Windows and an appveyor.yml file was also added to
enable AppVeyor CI support to ensure this doesn't regress.

cc #155