Add bank_account_authorized_at to subscriptions

[cbarton]

For back dating bank account authorization when importing subscriptions, the API user can specify a bank_account_authorized_at timestamp of when that authorization occurred.

cc/ @drewish

tests:

$account = Recurly_Account::get('<account_code>');
$sub = new Recurly_Subscription();
$sub->plan_code = '<plan_code>';
$sub->currency = 'USD';
$sub->account = $account;
$sub->bank_account_authorized_at = '2015-02-15T08:00:00+00:00';
$sub->create();

$sub = Recurly_Subscription::get($sub->uuid);
var_dump($sub->bank_account_authorized_at); //> 2015-02-15 08:00:00.000000"

[drewish]

+1