increment version number and updated changelog

git-svn-id: https://plugins.svn.wordpress.org/post-list-featured-image/trunk@962849 b8457f37-d9ea-0310-8a92-e5e31aec5664
changed: stable tag to 0.5.1

git-svn-id: https://plugins.svn.wordpress.org/post-list-featured-image/trunk@962853 b8457f37-d9ea-0310-8a92-e5e31aec5664