update to 2.9

Add support for old-style single use widgets

Bugfix

git-svn-id: https://plugins.svn.wordpress.org/jetpack-widget-visibility/trunk@868375 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme

git-svn-id: https://plugins.svn.wordpress.org/jetpack-widget-visibility/trunk@868376 b8457f37-d9ea-0310-8a92-e5e31aec5664
minor improvements

git-svn-id: https://plugins.svn.wordpress.org/jetpack-widget-visibility/trunk@869527 b8457f37-d9ea-0310-8a92-e5e31aec5664