Move css to assets folder, fix missing image in css.
Create a class ThemeCheckMain, fixes duplicate function load_styles reported in forums, plus looks better.

git-svn-id: https://plugins.svn.wordpress.org/theme-check/trunk@920047 b8457f37-d9ea-0310-8a92-e5e31aec5664
remove old style.css file

git-svn-id: https://plugins.svn.wordpress.org/theme-check/trunk@920048 b8457f37-d9ea-0310-8a92-e5e31aec5664