Added Grunt compress task and added to watch task

Added grunt-contrib-compress to your Gruntfile. It'll compress the dev and prod CSS produced by cssmin. Works with the compress task and with grunt watch.

grunt compress
  www/css/style-dev.min.css -> www/css/style-dev.min.css.gzip
  www/css/style-pro.min.css -> www/css/style-pro.min.css.gzip