Updated SubscriptionAddOnList.Add(AddOn, quantity) to address Issue #37

The Issue https://github.com/recurly/recurly-client-net/issues/37 stipulates that the Subscription price is overriding the Add On price when importing a Plan Add On in to a Subscription.

These changes update the SubscriptionAddOnList.Add(AddOn, quantity) logic to pull the UnitAmountInCents from the given AddOn. If the AddOn does not have a UnitAmountInCents for the given Subscription's currency, a ValidationException is thrown.

I have also included tests on SubscriptionAddOnList.Add(AddOn, quantity), covering a failure and success of importing the price, and data integrity through the import.