* Changes to thumbnail cache, converting 'created_timestamp' to just 'timestamp' and now storing this value for failed thumbnail generations.
* Minor tweaks to thumbnail table to use simpler array notation when accessing the 'thumber' section of the DG options.

git-svn-id: https://plugins.svn.wordpress.org/document-gallery/trunk@957018 b8457f37-d9ea-0310-8a92-e5e31aec5664