Properly detect HTML charset even if not reported by server

We need proper HTML charset detection, like it was done for XML.
Required to fix errors like http://trac.xbmc.org/ticket/14686#comment:8
Detection was implemented by following current HTML5 standard (works fine with early HTML versions).
Now CScraperUrl should correctly detect charset in most cases. No more workaround for umlauts and other non-US-ASCII chars.