Make column sortable

git-svn-id: https://plugins.svn.wordpress.org/show-dimensions-in-library/trunk@979621 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated readme.txt

git-svn-id: https://plugins.svn.wordpress.org/show-dimensions-in-library/trunk@979625 b8457f37-d9ea-0310-8a92-e5e31aec5664