Converted category selection to a hierarchal output

git-svn-id: https://plugins.svn.wordpress.org/simple-links/trunk@945190 b8457f37-d9ea-0310-8a92-e5e31aec5664
Bumping to version 2.5.6

git-svn-id: https://plugins.svn.wordpress.org/simple-links/trunk@945192 b8457f37-d9ea-0310-8a92-e5e31aec5664