Add CreditMemo model / service
Merge pull request #15 from huoxito/credit-memo

Add CreditMemo model / service
initial model / service for Bill Entity
ignored .ruby-* for RVM environments
fixed Bill#txn_date
added Model::BaseReference for ReferenceType objects
refactored Bill LineItem into its own class
added Bill rspec
added BillPayment entity with spec
added BillPayment service
Merge pull request #18 from sequielo/entity-bill-payment

Added support for BillPayment
All reference types, e.g. Account#parent_ref are implemented via a first-class `BaseReference` instance.