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
I'll switch it to `.button` so it matches up with what we have in https://github.com/jquery/jquery-ui/blob/master/themes/base/button.css