Initial changes to support moving logging away from native PHP error log to custom logging solution. Includes:
* Most of the necessary backend changes (nothing is known to be missing, but things always show up)
* Minor front-end changes to support enabling/disabiling logging from the advanced tab

git-svn-id: https://plugins.svn.wordpress.org/document-gallery/trunk@971551 b8457f37-d9ea-0310-8a92-e5e31aec5664