html-report CSS

In addition to https://github.com/danmar/cppcheck/pull/185 (which I merged into this pull request), this contains some other small fixes and improvements to the html-report page (which I noted in the commit message, I'm too tired to sum this up here right now, sorry)

http://i.imgur.com/MjjXkko.png -> http://i.imgur.com/ZQCovS2.png
http://i.imgur.com/0CJxMcG.png -> http://i.imgur.com/pqAWBmd.png