Add missing properties to Billing Info

[aaron-junot]

The properties braintree_payment_nonce and roku_billing_agreement_id were missing from the writable attributes of the billing info class. There were also a few property declarations missing from the PHPDocs so I went ahead and added those in as well.

script:

<?php

try {
  $billing_info = new Recurly_BillingInfo();
  $billing_info->account_code = 'example-1';
  $billing_info->first_name = 'Verena';
  $billing_info->last_name = 'Example';
  $billing_info->braintree_payment_nonce = 'fake-valid-nonce';
  $billing_info->roku_billing_agreement_id = 'a7b07fcc7763598bbf0387856d419047';
  $billing_info->create();

  print "Billing Info: $billing_info";
} catch (Exception $e) {
  print "$e";
}

[bhelx]

👍