Add a "last packaged" timestamp to {project_release_nodes}

As per the thread over at http://drupal.org/node/109181 it would be nice for project maintainers to see the time that the packaging script last tried to do anything for a given release node. That's almost what the "Last updated: August 19, 2007 - 16:45" timestamp is when you view a release node (e.g. http://drupal.org/node/168769) but it's not clear that's what we're displaying from the label of that field. Plus, I don't remember now if that's the only thing that touches that timestamp.

So, either we should:

A) verify the packaging script is the only thing that touches "Last updated", and if so, rename the label

or

B) Just add a new column to the {project_release_nodes} table, update it in package-release-nodes.php, and display it when viewing a release node.