Add credits to edit image dialog

git-svn-id: https://plugins.svn.wordpress.org/media-credit/trunk@992255 b8457f37-d9ea-0310-8a92-e5e31aec5664
Release 2.3.0

git-svn-id: https://plugins.svn.wordpress.org/media-credit/trunk@992257 b8457f37-d9ea-0310-8a92-e5e31aec5664