Add Serbian Translation

git-svn-id: https://plugins.svn.wordpress.org/black-post-slider/trunk@994148 b8457f37-d9ea-0310-8a92-e5e31aec5664
small fix in readme

git-svn-id: https://plugins.svn.wordpress.org/black-post-slider/trunk@994150 b8457f37-d9ea-0310-8a92-e5e31aec5664