Make subscription payment available through Subscription#payment

This commit grants access to another missing field in the subscription (besides the `next_capture_at` field on #24): the payment. Both fields are needed so you don't have to access subscription data via meta-programming, like you have to do right now:

```ruby
#...
data = {
  next_payment_at:  subscription.instance_variable_get('@next_capture_at'),
  card_type:        subscription.instance_variable_get('@payment')['card_type'],
  expiration_month: subscription.instance_variable_get('@payment')['expire_month'],
  expiration_year:  subscription.instance_variable_get('@payment')['expire_year']
}
```