commenting out CDATA for scripts and styles in XHTML documents

documents with XHTML mime type don't escape `<![CDATA` and `]]>` for inline scripts and styles as suggested by whatwg and MDC recommendations:

Patch adds
 - for scripts: `//<![CDATA` and `//]]>`
 - for styles: `/*<![CDATA*/` and `/*//]>*/`

References:
 - [whatwg: HTML vs. XHTML > Element-specific parsing
 > script and style](http://wiki.whatwg.org/wiki/HTML_vs._XHTML#Element-specific_parsing)
 - [MDC: Properly Using CSS and JavaScript in XHTML Documents](https://developer.mozilla.org/en-US/docs/Properly_Using_CSS_and_JavaScript_in_XHTML_Documents#Using_CDATA_Instead_of_Comments)

Pull request was made against `master` branch, but should be fixed in `2.5.x` as well

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30404