option to display for non-logged-in users only; code optimizations; use 'uid' or 'userid' as the Feedburner userid parameter name; plugin settings page is now tabbed; responsive CSS for mobile devices

git-svn-id: https://plugins.svn.wordpress.org/feedburner-optin-form/trunk@913623 b8457f37-d9ea-0310-8a92-e5e31aec5664