WooCommerce Store Credit 4.5.6

* Feature - Include the 'Store credit' used in the order totals.
* Feature - Display the 'Store credit' used in the invoices.
* Tweak - Exclude the 'Store credit' used from the discount order total.

* Feature - Rewritten the way the Store Credit coupons are applied.
* Tweak - Save the used store credit on each purchase.
* Tweak - Define the constants if not already set.
* Fix - Fixed incorrect 'Store Credit' discounts when applied in combination with other coupons.
* Fix - PHP notice for undefined index.
* Fix - Remaining credit amount not correct when using taxes.
* Fix - Removed the use of the third parameter in the 'array_filter' function (Require PHP 5.6+).
* Dev - Added constant 'WC_STORE_CREDIT_VERSION'.

* Fix - Partial use of store credit shows entire value of store credit as used.