Make U16InputStream an abstract interface for U16TrivialInputStream and U16ConverterInputStream

(U16TrivialInputStream, U16ConverterInputStream): New
(HTMLTokenizer::HTMLTokenizer): Take a pointer to U16InputStream as an argument instead of HTMLInputStream
(CSSInputStream, HTMLInputStream): Extend from U16ConverterInputStream
(HTMLScriptElementImp::execute): Refine
(toUpper) : New
Return the tagName attribute in ASCII uppercase in HTML documents.

(ElementImp::getTagName): Just return nodeName as the tag name
(ElementImp::ElementImp): Generate the tag name to nodeName
(DOMImplementationImp::createDocument): Create the normal document for now; add a TODO
(DocumentImp::createElement): Refine