Fixed variable error in wpsc-shipping/usps_20.php

The variable being tested is NOT the one being assigned the new instance of ASHTools().
Filter gateway checkout form output
Add docblocks for all functions in gateways.php
Merge pull request #210 from jbeales/issue-209

Fix for #209
Limit quantity of individual products that can be added to the cart. Default value is filterable.
Limit product quantities @since 3.8.10 and documentation formatting.
Ensure all variations are deleted when deleting a product.
Fix split() deprecation notice in PHP 5.3+
Merge branch 'master' of https://github.com/wp-e-commerce/WP-e-Commerce
Replace plus-minus.png with CSS.
Added new coupon display files
Improved formatting
Link to new edit view
Remove coupon edit form
remove ending ?>
Initial edit screen rework
Finished up initial edit screen rebuild
Progress towards making conditions work
replace filter with action
Added edit_coupon_is_used input