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