Merge pull request #195 from sortega/issues/41

Make client.exchange.Handshake and common.exchange.Handshake more similar
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