updated add fee function

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-extra-fee-option/trunk@788245 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated css and add pro version link

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-extra-fee-option/trunk@791221 b8457f37-d9ea-0310-8a92-e5e31aec5664