CssMin does not preserve license comments

[kittsville]

While JShrink is capable to being configured to preserve comment blocks starting with /*!, thus allowing for licensing information to be preserved, it seems CssMin cannot. This is not good seeing as removing that stuff is illegal/immoral. I'm pretty sure nobody but me has ever visited WP-Librarian's repo page, let alone the Dev Kit's page, but that doesn't mean I should start dicking with other people's code.

To keep things legal in the meantime I've set CssMin to preserve all commens (see 123b82e). While this'll add a minor overhead it doesn't really matter because WP-Librarian is still a long way from a production release.

[kittsville]

Marked as long term because I probably won't bother trying to fix this for a long time.