* Created PaymentData transfer objects for (recipient, assetId, amount) tuples
* Created corresponding Payment class for validating, processing and orphaning payment(s)
* Modified OrderData to support isClosed for when an Order is cancelled so no more trades can occur
* Migrated CancelOrderTransactions and MultiPaymentTransactions
* Converted MessageTransactions, PaymentTransactions and TransferAssetTransactions to use new Payment class
Can't use PaymentTransformer in PaymentTransformer or TransferAssetTransformer due to serialization differences.
