Back port some code fixes for CSS optimizations

Going to postpone this until a content attribute patch passes tests.

Updated issue summary.

Committed this patch.

Will need to test the @import stuff as this line is quite different from where the patch was left

  return preg_replace('%url\(\s*+([\'"]?+)(?![a-z]++:|/)([^\'")]+)([\'"]?+)\s*\)%i', 'url(\1' . $directory . '\2\3)', $file);

Committed some core tests to make sure the import stuff works. It does :)