GiveWP - 2Checkout Gateway 2.0.0

* Fix: Ensure the donor's first and last names are properly sent to the 2Checkout gateway.
* Fix: If switching gateways ensure that 2Checkout continues to function properly if a validation error occurs.
* Tweak: Update the 2Checkout PHP SDK to the latest version.

* Fix: Resolved error when Give is not activated and 2Checkout remains active.

* New: Added the ability to process refunds directly within the payment gateway. This will be only applicable to new
* Tweak: The plugin now respects its own constants. This is useful for non-traditional WP setups where plugins may reside in a separate directory.
* Tweak: Changed the position notices display if a transaction is declined to be more noticable for donors.
* Fix: If an incorrect CVC is entered it is now properly validated.