Working code examples for std::rt::io::net

Hi!

I was trying to play with networking code today, and noticed the code example in std::rt::io was not really corresponding to TcpStream.

If that code is ok, I'll add more examples in this pull request for TcpStream and UdpStream, client and server and squash it in one commit.

BTW, it seems that std::io::{Reader,Writer} and std::rt::io::{Reader,Writer} are not consistent. These code examples would be nicer if I could use std::io::{ReaderUtil,WriterUtil}. Should I continue the work that began in #9749 to make them available in std::rt::io?