Gruntfile.js: add task cssmin, to minify the flexslider.css
flexslider.css: a special seperate file, meant to overide the slider css
flexslider.css.min: minified css
package.json: adding grunt-contrib-cssmin
header.php: this file was added from parent theme, modified from the original by placing a special link tag to load in the css that overrides the plugin css. This is needed because by default, the plugin css is included after style.css and presents a problem with css precedence.