private boolean isUrl(String urlString) {
		try {
			new URL(urlString);
			return true;
		} catch (MalformedURLException mue) {
			return false;
		}
	}
check whether input is a valid URL

@author bob.li.0718@gmail.com
@param urlString urlString
@return true means yes, otherwise no.
webmagic-selenium/src/main/java/us/codecraft/webmagic/downloader/selenium/WebDriverPool.java