Add interface for PeerActor
Add replace BlockchainActor with PeerActor in ExchangeActor
Add last offer querying to MicropaymentChannel actors
Add ExchangeTransactionBroadcastActor interface
Add scaladoc to exchange actor
Add height notification capability to blockchain actor
Add constant value actor
Implement ExchangeTransactionBroadcastActor
PR feedback
Merge pull request #207 from edrevo/add-tx-broadcast-actor

Add interfaces for transaction broadcast actor and bitcoin's peer actor