Add a warning when the WordPress directory isn't writeable, and the user hasn't set their S/FTP wp-config settings.

git-svn-id: https://plugins.svn.wordpress.org/automatic-updater/trunk@743854 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added AUTOMATIC_UPDATER_DISABLED wp-config option, for forcing updates to never happen

git-svn-id: https://plugins.svn.wordpress.org/automatic-updater/trunk@743856 b8457f37-d9ea-0310-8a92-e5e31aec5664