Support deserialization of a pending subscription.

* <subscription> element may have a child <pending_subscription> element when subscription changes are scheduled at renewal.
Support Coupon Redemption API

Recurly API specification is described in
 http://docs.recurly.com/api/coupons/coupon-redemption

* RecurlyClient.redeemCoupon()
* RecurlyClient.getCouponRedemptionByAccount()
* RecurlyClient.getCouponRedemptionByInvoice()
* RecurlyClient.deleteCouponRedemption()

Also implemented coupon deactive API for unit testing.

* RecurlyClient.deleteCoupon()
Merge pull request #26 from oohira/support-pending-subscription

Support deserialization of a pending subscription.
Fix minor problems about Coupon Redemption API

* Change method signature of RecurlyClient.getCouponRedemptionByInvoice(String) to RecurlyClient.getCouponRedemptionByInvoice(Integer)
* Fix copy and paste mistake in Redemption.equals()
Merge pull request #27 from oohira/support-coupon-redemption

Support Coupon Redemption API
test: fix intermittent tests failures

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>