added Adjustments.Get, added new QueryStringBuilder to simplify logic for lists, added abstract RecurlyEntity to restrict List implementations, made pluralized utility access classes sealed
added Adjustment, BillingInfo, and coupon snippets
added TaxExempt property on Plan
snippets for Coupon Redemptions through Plan Add ons
missed one on plan add ons
added missing Subscription.Preview implementation
added missing subscription add on impl
added a convenience property to full enumerate a RecurlyList
added subscription snippets'