Tweak TaskIo wording and such

* Remove TaskIo
* task_split -> split
* TaskIoRead -> ReadHalf
* TaskIoWrite -> WriteHalf

Closes #18

tests/echo.rs

@@ -8,7 +8,7 @@ use std::thread;
 
 use futures::Future;
 use futures::stream::Stream;
-use tokio_core::io::{copy, TaskIo};
+use tokio_core::io::{copy, Io};
 use tokio_core::net::TcpListener;
 use tokio_core::reactor::Core;
 
@@ -42,7 +42,7 @@ fn echo_server() {
 
     let clients = srv.incoming();
     let client = clients.into_future().map(|e| e.0.unwrap()).map_err(|e| e.0);
-    let halves = client.map(|s| TaskIo::new(s.0).split());
+    let halves = client.map(|s| s.0.split());
     let copied = halves.and_then(|(a, b)| copy(a, b));
 
     let amt = t!(l.run(copied));