split Client into (Client, Connection) (#107)

The Connection type is a `Future` that drives all of the IO of the
client connection.

The Client type is separate, and is used to send requests into the
connection.

tests/ping_pong.rs

@@ -8,7 +8,9 @@ fn recv_single_ping() {
     let (m, mock) = mock::new();
 
     // Create the handshake
-    let h2 = Client::handshake(m).unwrap().and_then(|conn| conn.unwrap());
+    let h2 = Client::handshake(m)
+        .unwrap()
+        .and_then(|(_, conn)| conn.unwrap());
 
     let mock = mock.assert_client_handshake()
         .unwrap()