Switch over to the Shipping Zones functionality

WC 2.5 added shipping zones, which allow multiple instances of the same plugin, and also already provides geographical restrictions.

• Read up on Shipping Zones
• Implement shipping zones in general
• enable instance-based settings
• Figure out a way to migrate global settings to zone-based settings
• Or find a way to provide a legacy method class that uses the old, global settings if the plugin was used in the past (see class-wc-shipping.php, function get_shipping_method_class_names)
• Add rule syntax help in the method's config pages
• Add upgrade nag box in method's config pages
• Update documentation on the homepage
• Update screenshots at http://wordpress.org/plugins/