Improved item change logging
* Entries are made via the ProdUpdateModel class. The helper method
  logUpdate() simplifies the process. Using the class provides flexibility
  to add columns without breaking other code
* The save() method in ProductModel automatically logs changes. Ideally
  this will reduce situations where the developer forgets to make an
  entry in the update history.
* Deletions are now logged
* The nightly.batch.php job needs to log as well but I haven't
  updated that yet.