Add io::lines

[oberien]

As discussed in the Chat example PR it would be nice to have a function in tokio-core, which takes a BufRead and returns Stream<Item=String, Error=io::Error>. This would make it easy to read lines asynchronously from a TcpStream/Read.

On the other hand side, there is tokio-line, a repository based on the idea of line-based asynchronous handling. Adding such a method to tokio-core would definitely interfere with the idea of tokio-line.

/cc @alexcrichton @carllerche

[alexcrichton]

Done in https://github.com/alexcrichton/tokio-io, the migration of this crate will be tracked by #61