add option to allow customisation of no events message

git-svn-id: http://plugins.svn.wordpress.org/google-calendar-events@678637 b8457f37-d9ea-0310-8a92-e5e31aec5664
prevent notice on uninstallation when there are no feeds

git-svn-id: http://plugins.svn.wordpress.org/google-calendar-events@678639 b8457f37-d9ea-0310-8a92-e5e31aec5664