preserve comments in less if 'compress' config disabled

Otherwise comments are never visible in css.php
Corrected compression for ++ and -- operator. Partially fixes #897.
Merge pull request #896 from splitbrain/lessmorecomments

preserve comments in less if 'compress' config disabled
Merge pull request #901 from LarsGit223/master

Corrected compression for ++ and -- operator. Partially fixes #897.