refactor(lib): convert usage of tokio_core::io to tokio_io

This commit updates to the most recent versions (released today) of the various
Tokio libraries in use. Namely the `tokio_core::io` module has now been
deprecated in favor of an external `tokio-io` crate. This commit pulls in that
crate and uses the `AsyncRead + AsyncWrite` abstraction instead of `Io` from
tokio-core.

BREAKING CHANGE: Any external types that were using that had implemented `Io` will need to 
  implement `AsyncRead + AsyncWrite` from tokio_io.

src/mock.rs

@@ -1,8 +1,8 @@
 use std::cmp;
 use std::io::{self, Read, Write};
 
-use futures::Async;
-use tokio::io::Io;
+use futures::Poll;
+use tokio_io::{AsyncRead, AsyncWrite};
 
 #[derive(Debug)]
 pub struct Buf {
@@ -123,21 +123,12 @@ impl<T: Write> Write for AsyncIo<T> {
     }
 }
 
-impl<T: Read + Write> Io for AsyncIo<T> {
-    fn poll_read(&mut self) -> Async<()> {
-        if self.bytes_until_block == 0 {
-            Async::NotReady
-        } else {
-            Async::Ready(())
-        }
-    }
+impl<T: Read + Write> AsyncRead for AsyncIo<T> {
+}
 
-    fn poll_write(&mut self) -> Async<()> {
-        if self.bytes_until_block == 0 {
-            Async::NotReady
-        } else {
-            Async::Ready(())
-        }
+impl<T: Read + Write> AsyncWrite for AsyncIo<T> {
+    fn shutdown(&mut self) -> Poll<(), io::Error> {
+        Ok(().into())
     }
 }