Support web pages using non-utf8 charset

[philipjkim]

For example:

func bodyStr(res *http.Response) (s *string, err error) {
    defer res.Body.Close()
    body, err := ioutil.ReadAll(res.Body)
    if err != nil {
        return nil, fmt.Errorf("ioutil.ReadAll failed: %v", err)
    }
    result := string(body)
    _, cs, _ := charset.DetermineEncoding(body, res.Header.Get("Content-Type"))
    if cs == "windows-1252" {
        cs = "euc-kr"
    }
    if cs != "utf-8" {
        result, err = iconv.ConvertString(result, cs, "utf-8")
        if err != nil {
            return nil, fmt.Errorf("Converting string from %v to utf-8 failed, url: %v",
                cs, res.Request.URL.String())
        }
    }
    return &result, nil
}