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/support/prelude.rs

@@ -60,7 +60,7 @@ pub trait ClientExt {
     fn run<F: Future>(&mut self, f: F) -> Result<F::Item, F::Error>;
 }
 
-impl<T, B> ClientExt for Client<T, B>
+impl<T, B> ClientExt for client::Connection<T, B>
 where
     T: AsyncRead + AsyncWrite + 'static,
     B: IntoBuf + 'static,