V2 rc1

- renamed `account.CreateAdjustment()` -> `account.NewAdjustment()` to better describe that a call to the Recurly API does not occur.

- renamed `plan.CreateAddOn()` -> `plan.NewAddOn()` to better describe that a call to the Recurly API does not occur.

- added; multiple adjustment refunds support `invoice.Refund(invoice.Adjustments)`

- added; default timeframe support to `subscription.ChangeSubscription()`
`Subscription.TimeFrame.Now` is the default and no longer is required to be passed.

- added; overloads to `subscription.AddOns.Add()`. now supports:
  - subscription.AddOns.Add("plan addon code");
  - subscription.AddOns.Add(planAddOn);
  - subscription.AddOns.Add(new SubscriptionAddOn(..)); // no change

- added support for all Sales Tax API additions

- renamed `Recurly.Exception` -> `Recurly.RecurlyException` to play nicer with `System.Exception` #21 

- changed `Recurly.RecurlyException` to inherit from `System.Exception` instead of `System.ApplicationException` #21