Closed #1498 Added shopp_stock_product action hook

Added ProductSummary::rebuild() to make it easier to mark a specific product for summary recalculation

Refactored the ShoppAjax::update_inventory() handler to use shopp_product_variant_set_stock()
Adds the ability for negative postcode matching conditions in the Shipping modules. Syntax for negative conditions is a ! at the very begining of the whole matching set.
Fixed credits so they load the first time and so contributor profiles are linked properly
Refactored exclusive postcode handling and added support for individual rule exclusions by adding '!' to the beginning of the rule.
Replaced anonymous function with create_function for PHP 5.2 support and cleaned up variable names.
Merge pull request #2417 from msigley/patch20130912

Negative postcode matching in shipping modules
#2275 Code formatting cleanup