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())
             }

src/http/h1/parse.rs

@@ -55,7 +55,7 @@ impl Http1Transaction for ServerTransaction {
         };
 
         let mut headers = Headers::with_capacity(headers_len);
-        let slice = buf.drain_to(len).freeze();
+        let slice = buf.split_to(len).freeze();
         let path = slice.slice(path.0, path.1);
         // path was found to be utf8 by httparse
         let path = unsafe { ByteStr::from_utf8_unchecked(path) };
@@ -171,7 +171,7 @@ impl Http1Transaction for ClientTransaction {
         };
 
         let mut headers = Headers::with_capacity(headers_len);
-        let slice = buf.drain_to(len).freeze();
+        let slice = buf.split_to(len).freeze();
         headers.extend(HeadersAsBytesIter {
             headers: headers_indices[..headers_len].iter(),
             slice: slice,