Make successURL change backwards compatible, deprecate static interface

This release allows you to use the old, deprecated backURL param and the library will automatically convert it to successURL.

Also the static interface has been deprecated:

Instead of

FlexPay::get_purchase_URL("flexpay secret", [...]);

Use:

$brand = Verotel\FlexPay\Brand::create_from_merchant_id(/* Your customer ID */ '9804000000000000');
$flexpayClient = new Verotel\FlexPay\Client(/* shop ID */ 12345, "FlexPay Signature Key", $brand);

$purchaseUrl = $flexpayClient->get_purchase_URL([
    "priceAmount" => 2.64,
    "priceCurrency" => "EUR",
    "description" => "Test purchase",
]);