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()
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