add websocket support to /container/<name>/attach/ws

This function add the possibility to attach containers streams to a
websocket. When a websocket is asked the request is upgraded to this
protocol..

Example of usage:

    package main

    import (
        "fmt"
        "code.google.com/p/go.net/websocket"
    )

    func main() {
        origin := "http://localhost/"
        url :=
        "ws://localhost:4243/v1.3/containers/450a8d4a34ee/attach/ws?logs=1&stderr=1&stdout=1"
        ws, err := websocket.Dial(url, "", origin)
        if err != nil {
            fmt.Printf("error: %s", err)
            return
        }


        for {
            msg := make([]byte, 500)
            var n int
            if n, err = ws.Read(msg); err != nil {
                break
            }
            fmt.Printf("%s.\n", msg[:n])
        }
    }