clarify some code

git-svn-id: https://plugins.svn.wordpress.org/advertising-manager/trunk@951890 b8457f37-d9ea-0310-8a92-e5e31aec5664
rewrite list to use wordpress list table

git-svn-id: https://plugins.svn.wordpress.org/advertising-manager/trunk@951891 b8457f37-d9ea-0310-8a92-e5e31aec5664