account.CreateAdjustment -> account.NewAdjustment

Create implies sending a POST to the Recurly API.
New implies creation in memory.
plan.CreateAddOn -> plan.NewAddOn

Create implies sending a POST to the Recurly API.
New implies creation in memory.
remove todo
Adjustment; permit setting properties
Account; add missing vat and tax exempt props
tests; updating Account with tax & vat
Account; TaxExempt should allow null
Plan; test TaxExempt
improved debugging
fixed; returning all invoices