Adding Demur's recommendations for giving more flexibility with sanitization of post_type and post_status attributes.

git-svn-id: http://plugins.svn.wordpress.org/document-gallery/trunk@1044102 b8457f37-d9ea-0310-8a92-e5e31aec5664
Tested up to 4.0.1
Added limit, post_type, and post_status to the changelog version 2.3 description

git-svn-id: http://plugins.svn.wordpress.org/document-gallery/trunk@1044104 b8457f37-d9ea-0310-8a92-e5e31aec5664