82 -  CSS style guide

A proposed style guide to cover CSS code commits to the jQuery projects.  Open to suggestion for rules, this pass is based on the CSS in jQuery Mobile and jQuery UI.

Based off of Issue https://github.com/jquery/contribute.jquery.org/issues/82
Either also use singular "rule declaration" here, as in the line above, or use "declaration blocks" in both cases, or whatever else, as long as its consistent.