Remove uv I/O from extra.

[brson]

This implementation is deprecated in favor of std::rt::io.

[gavinb]

I've made a start on this issue. I removed timer.rs, uv.rs, uv_global_loop.rs, uv_iotask.rs, and uv_ll.rs from libextra. Then went about fixing the breakage. The modules net_ip.rs and net_tcp.rs depend on this support, and then net_url.rs and net.rs which depend on them. Should they removed also? Or should they be converted to use the uv support in libstd?

[brson]

@gavinb I think net_url is the only thing we want to keep. There's an IP type in std::rt::io::ip (or somewhere) that doesn't depend on uv that can replace the libextra IP type.