Change default XPath selector from HtmlCleaner to Xsoup.

Xsoup is an XPath selector based on Jsoup written by me. It has much better performance than HtmlCleaner.

Time of processing a page is reduced from 7~9ms to 0.4ms.

If Xsoup is not stable for your usage, just use Spider.xsoupOff() to turn off it and report an issue to me!

Add cycle retry times for Site.

When cycle retry times is set, Spider will put the url which downloading failed back to scheduler, and retry after a cycle of queue.