Add test infrastructure to work directly with frames (#56)

This adds a `Codec` based testing API. This is a bit less annoying than writing
at the raw H2 wire protocol level...

src/client.rs

@@ -57,6 +57,9 @@ impl<T, B> Client<T, B>
     ///
     /// Returns a future which resolves to the connection value once the H2
     /// handshake has been completed.
+    ///
+    /// It's important to note that this does not **flush** the outbound
+    /// settings to the wire.
     pub fn handshake2(io: T) -> Handshake<T, B> {
         use tokio_io::io;