Support deserialization of a pending subscription.

When subscription changes are scheduled at renewal (not immediately), `<subscription>` element has a child `<pending_subscription>` element, though Recurly API Document refers this only slightly (see http://docs.recurly.com/api/subscriptions).

I confirmed following xml was received from Recurly.

```xml
<subscription href="https://api.recurly.com/v2/subscriptions/...">
  ...
  <pending_subscription type="subscription">
    <plan href="https://api.recurly.com/v2/plans/XXXX">
      <plan_code>XXXX</plan_code>
      <name>Some plan name</name>
    </plan>
    <unit_amount_in_cents type="integer">1900</unit_amount_in_cents>
    <quantity type="integer">1</quantity>
    <subscription_add_ons type="array">
    </subscription_add_ons>
  </pending_subscription>
```

Because I want to handle pending subscriptions, I implemented deserialization logic of a `<pending_subscription>` element. Please review and merge this modification if possible.

Thanks