Fixed duplicate script listing and add source line and some more small changes

PHP:
- Fix: duplicate script listings
- Added: non-obtrusive script/style source line
- Small adjustments to the html to be inline with the general debug bar semantics and styling

CSS:
- Moved table row highlighting to CSS
- Some other minor adjustments to compensate for the twentythirteen theme screwing up the table display on the front-end

Readme:
- Update changelog
- Updated stable tag as otherwise (when added to the WP SVN) no upgrade will be offered