Replace validateRefundSignature with signMyRefund
Introduce StepSignatures in client.exchange.Handshake

And replace more implementation details with stuff of common.exchange
Introduce client.handshake.DefaultExchangeProtocol

To make both DefaultHandshake classes almost identical
Remove exchange and role attributes from Handshake
Completely replace client.exchange.Handshake
Merge both DefaultHandshakeTest classes
Merge pull request #199 from sortega/issues/41

Replace old Handshake by the new one