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/http/h1/decode.rs

@@ -295,7 +295,7 @@ mod tests {
                 let (a, b) = self.split_at(n);
                 let mut buf = BytesMut::from(a);
                 *self = b;
-                Ok(buf.drain_to(n).freeze())
+                Ok(buf.split_to(n).freeze())
             } else {
                 Ok(Bytes::new())
             }