Replaces TODO with upgrade-friendly migration for adding return_label_im...

...age to shipping_shipment_package table.

Made possible by Varien_Db_Adapter_Mysqli's `addColumn`, which checks for an existing column of the same name before actually adding it.

https://github.com/LokeyCoding/magento-mirror/blob/magento-1.7/lib/Varien/Db/Adapter/Mysqli.php#L240

Tested against 1.7.0.2 with no problems. `addColumn` hasn't changed since 1.1.1 (circa 2009) so this change should be safe for most any Magento installation.