Change HTMLConverter to parse and render HTML5

Currently, the default implementation of the HTMLConverter used by the WYSIWYG editor uses XHTML 1.0. For CKEDITOR-207, this needs to be changed to HTML 5.0. Note that the proposal here is to simply change the default implementation and not to implement a separate HTML5 implementation as HTML5 is the future and our HTML5 renderer and future parser are so similar to XHTML 1.0 that we do not expect this will break anything.