Add gateway_code to subscription and invoice

[aaron-junot]

Allows gateway_code to be updated via the subscription/notes endpoint and the PUT invoice endpoint

Scripts:

<?php
// Assumes subscription uuid "46c7ddb06e35530669baa445e3926e34" exists
include './scripts/php/stub.php';

$uuid = '46c7ddb06e35530669baa445e3926e34';
$sub = Recurly_Subscription::get($uuid);
$sub->updateNotes(array("gateway_code" => "jvkvq2er456m");
print("\n");

$invoice = Recurly_Invoice::get('1001');
$invoice->gateway_code = 'jvm7ol289raz';
$invoice->update();

[drewish]

Looks good but will wait for the WIP label to come off before merging.

[aaron-junot]

Oh yeah, I'm going to go ahead and put the similar change to the Invoice object on this same PR and do a test. I'll ask you to take another look when I take off the WIP label

[bhelx]

👍