Adding parseDomForCharsetDetection interface method in GadgetHtmlParser

Adding a parseDomForCharsetDetection method separate from parseDom method. The objective is to parse out some portion of the initial html to be able to parse the <meta http-equiv> tag for charset encoding detection.

This method can do an approximate job of parsing the content as it is only going to be called when we want to detect the charset encoding of the document.

FYI:
For more robust charset detection, we should think of implementing the encoding sniff algo described here:
http://www.whatwg.org/specs/web-apps/current-work/multipage/urls.html#content-type
http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html