Publish a rules event when stock level changes for a product

Problem/Motivation

I want to be able to perform actions on nodes when the stock level changes for a product, for example unpublishing a node that has zero stock.

Proposed resolution

Add a rule event that fires when stock levels change, so that I can then use the rules framework to act on it.

Remaining tasks

Attached is code that performs this. It's currently designed as a separate module, but probably should be integrated into stock core.

User interface changes

None?

API changes

I have encountered two problems with building this module:

1. There is no uc_product entity. This would have made most sense for what to publish when stock level changes. Using the uc_order_product entity didn't work because uc_order_product requires a uc_order entity to associate itself with (and gives a crypt "Access denied!" error when you don't have a uc_order context).
2. The uc_quote module has a minor problem if there is an update to a product during the ordering process. I got the following error when the node was unpublished during the checkout process: (Issue filed separately)
   
   

   Notice: Undefined property: stdClass::$street1 in uc_quote_node_update() (line 105 of /var/local/drupal7/sites/all/modules/ubercart/shipping/uc_quote/uc_quote.module).