Merge pull request #173 from sortega/issue/30_handshake

Refactor tests to be able to delegate subscriptions to delegate actors
Convert GatewayProbe into MockGateway

Make GatewayProbe more active by using an actor providing a fake
implementation of the message gateway. From now on we can avoid tracking
how actors subscribe to messages (however we can keep doing so).
Stop tracking subscriptions on HandshakeActor tests
Rename protocol constant to reflect its current use
Add step information to PaymentProof message
Stop using MessageForwarding

It is going to be replaced by the MessageForwarder
Group handshake actor collaborator actor refs
Reorganize HandshakeActor initialization information

Data is now organized in several case classes that get passed as
constructor arguments.
Merge pull request #175 from edrevo/cf-49

Add step information to PaymentProof message
Simplify HandshakeActor by removing StartHandshake message
Fix subscription handling at MockGateway
Merge pull request #174 from sortega/issue/30_handshake

Prepare for the first use of MessageForwarder on HandshakeActor
Fix coinffeine-test compilation
Use MessageForwarder for peer handshake and getting refund signature
Add state diagrams for micropayment channel actors
Merge pull request #176 from sortega/issue/30_handshake

First uses of MessageForwarding on HandshakeActor
Merge pull request #177 from apoloval/task/ForwardThatDamnedMessages

Add state diagrams for micropayment channel actors
Add a new public message `MicropaymentChannelClosed`

This message will be used by the buyer to notify it decided to close the
micropayment channel (e.g., because it suceed :-) )
Extract an abstract class from `MockGateway`

This allows to implement other forms of mocked message gateways easily.
Fix buggy peer ID in `SampleExchange`