add screenshots

git-svn-id: https://plugins.svn.wordpress.org/wp-share-buttons/trunk@1027884 b8457f37-d9ea-0310-8a92-e5e31aec5664
Edit screens

git-svn-id: https://plugins.svn.wordpress.org/wp-share-buttons/trunk@1027888 b8457f37-d9ea-0310-8a92-e5e31aec5664