Remove is_admin check

git-svn-id: https://plugins.svn.wordpress.org/post-by-email-links/trunk@930951 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update README.txt

git-svn-id: https://plugins.svn.wordpress.org/post-by-email-links/trunk@930952 b8457f37-d9ea-0310-8a92-e5e31aec5664