add CSRF protection and sanitize user input

git-svn-id: https://plugins.svn.wordpress.org/simple-sticky-footer/trunk@1034365 b8457f37-d9ea-0310-8a92-e5e31aec5664
add wp_verify_nonce to form submit

git-svn-id: https://plugins.svn.wordpress.org/simple-sticky-footer/trunk@1034367 b8457f37-d9ea-0310-8a92-e5e31aec5664