Added re-order drag and drop

git-svn-id: https://plugins.svn.wordpress.org/wp-asset-manager/trunk@893797 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated README

git-svn-id: https://plugins.svn.wordpress.org/wp-asset-manager/trunk@893800 b8457f37-d9ea-0310-8a92-e5e31aec5664