Add SellerExchangeActor
Use context.stop instead of PoisonPill
PR feedback
Merge pull request #112 from edrevo/add-seller-exchange-actor

Add SellerExchangeActor
Make BuyerExchangeActor to receive initialization message
Make HandshakeActor to receive an initialization message
Make PeerActor to receive an initialization message
Make MessageGateway to receive an initialization message
Make HandshakeArbiterActor to receive an initialization message