Cron event for nodes

In fact, this patch is quite ugly. Nonetheless, I think it may be of use in some situations. But be warned.

For a customer we had to build a workflow which didn't just change states during user initiated events, but also on some temporal events, like when a certain date is passed. Therefore I needed an event to be triggered periodically so I could check some conditions. I saw no other way than to trigger an event on every node during a cron-run. That's what I've implemented and for this situation it works pretty good. As I said, it's ugly and inefficient, but it works.

This patch adds a cron-event to workflow-ng's node-integration called "node inspected by cron". Furthermore it extends the node module's hook_cron() so that the event is actually triggerred for every node.

So, I don't expect maintainers to include this code. I'm posting it just to share with people that might need it.