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
Rename client.exchange.Exchange to ProtoMicroPaymentChannel
Make exchange and role available on MicroPaymentChannelActor
Merge pull request #199 from sortega/issues/41

Replace old Handshake by the new one
Remove UserRole hierarchy
Move pay responsibility out of ProtoMicroPaymentChannel
Provide a pimp-my-library class for `Wallet`

This pimp-my-lib class extends the functionality of bitcoinj's `Wallet`
in order to:

* Have some accessors that returns `BitcoinAmount` instances rather than
`BigInteger` objects representing Satoshis.
* Have a function `blockFunds()` that may be used to block funds. This
will commit the tx marking its funds as spent.
* Have a function `releaseFunds()` that may be used to release
previously blocked funds.
Merge pull request #200 from apoloval/issues/49

Provide a pimp-my-library class for `Wallet`
Move payment validation from ProtoMicroPaymentChannel
Remove some payment-related leftovers
Merge pull request #201 from sortega/issues/41bis

Refactor client.exchange.Exchange to make it similar to MicroPaymentChannel
Completely remove ExchangeInfo
Replace HandshakeFactory for an ExchangeProtocol
Merge pull request #202 from sortega/issues/41

Replace ExchangeInfo and HandshakeFactory
Use exchange.amounts for consistency