support resuming of file transfers

[laole918]

        File path = new File("/Users/laole918/Downloads/HttpUtils");
        File meizi = new File(path, "meizi.jpg");
        String meiziUrl = "http://i.meizitu.net/2013/08/131Z5K41-0.jpg";
        int contentLength = HttpRequest.get(meiziUrl).contentLength();
        int unitLength = contentLength / 2;
        if (unitLength > 0) {
            int unitOverLength = contentLength % 2;
            for (int i = 0, j = 0; i < contentLength && j < 1; i += unitLength, j++) {
                int end = i + unitLength;
                HttpRequest.get(meiziUrl).range(i, (j == 1) ? end + unitOverLength : end).receive(meizi, i);
            }
        }