Enable listening for updates to BillingHandlers to enable rebinding event listeners
BillingHandlerEventListenerFactory to general EventListenerFactory for multiple listener types
Added addLineItemsToBill utility method to BaseBillAssociator
LatestPendingBillAssociator
NewBillingHandlerListener to update event binding according to changes to billing handlers
Added LatestPendingBillAssociatorResource and cleaned up SimpleNewBillAssociatorResource