Added response.Format(request, handlers) for content negotiation

[prabirshrestha]

Initial PR for #3 which is similar to expressjs res.format(obj)

Usage:

app.Get("/", func (w http.ResponseWriter, r *http.Request) {
    response.Format(r, map[string]func() {
        "text/html": func () {
            w.Header().Set("Content-Type", "text/html")
            fmt.Fprintln(w, "Hello world<br/>")
            fmt.Fprint(w, r.Header.Get("Accept"))
        },
        "application/json": func () {
            response.JSON(w, map[string]string { "hello": "world" })
        },
        "default": func () {
            fmt.Fprintf(w, "default")
        },
    })
})

Currently I'm internally using bitbucket.org/ww/goautoneg which is a mercurial repository. We should have it hosted in github or embed it in response repository.