add ergonomics to testing with frames

[seanmonstar]

• Add support::frames builders, to ease creating frames to compare with or send on the mocked IO.
• Add extension methods for futures that return a Handle, such as send_frame and recv_frame.

Ends up looking something like this:

let mock = srv.assert_client_handshake().unwrap()
    .recv_settings()
    .recv_frame(
        frames::headers(1)
            .request("GET", "https://http2.akamai.com/")
            .eos()
    )
    .send_frame(
        frames::headers(1)
            .response(200)
    )
    // send 2 data frames
    .send_frame(frames::data(1, &payload[..]))
    .send_frame(frames::data(1, &payload[..]).eos())
    // gotta end the connection
    .map(drop);

[carllerche]

Tests look much nicer.

It's a bit late for me to digest all the type related implementation detail, but merge it if you want to. We can always iterate on master.