PW WooCommerce Gift Cards Pro 3.1

- Improved compatibility with WordPress Multisite. Added config flag to share the database across sites: PWGC_MULTISITE_SHARED_DATABASE

- Changed the hook from woocommerce_thankyou to woocommerce_payment_complete to make this more compatible with payment processors that use the Thank You page even for failed orders. Added a new hook pw_gift_cards_amount_attribute_visible_on_product_page to optionally hide the Amount attributes on the product page.

- Updated hooks for status changes to make it more comptaible with WooCommerce POS.