client.Config describes how to make a client connection to a server
for HTTP traffic, but for connection upgrade scenarios cannot be
used because the underlying http.Transport object can't allow the
connection to be hijacked. Reorganize the TLS and connection wrapper
methods so that a sophisticated client can do:

    cfg := &client.Config{...} // from somewhere
    tlsConfig, _ := client.TLSConfigFor(cfg)
    _ := conn.Dial(...)
    rt := MyRoundTripper() // some func that implements grabbing requests
    wrapper, _ := client.HTTPWrappersFor(cfg)
    req := &http.Request{}
    req.Header.Set("Connection-Upgrade", ...)
    _, := wrapper.RoundTrip(req)
    // rt has been invoked with a fully formed Req with auth
    rt.Req.Write(conn)
    // read response for upgrade

It would be good to have utility function that does more of this,
but mostly enabling the HTTP2/SPDY client exec function right now.