use node_save instead of poking around in the database

This patch runs the node update trough the proper Drupal channels, by calling node_save when (un)publishing. This is a lot better then poking in the database for three reasons:
* It allows other modules (such as workflow) to hook into the event of updating. This is crucial.
* It is the right way to do it: node_save is there for exactly such events.
* It is future proof: we don't need to maintain database schemes.

One downside is that for larger amounts of nodes/events, this performs rather bad.