Merge pull request #202 from sortega/issues/41

Replace ExchangeInfo and HandshakeFactory
Use exchange.amounts for consistency
Move createMicroPaymentChannel to ExchangeProtocol
Remove unnecessary type parameters
Use MicroPaymentChannel.Step in ProtoMicroPaymentChannel
Remove unused and deprecated code
Use MicroPaymentChannel.Signatures elsewhere
Harmonize ProtoMicroPaymentChannel and MicroPaymentChannel traits
Use ExchangeProtocol as a factory of ProtoMicroPaymentChannel

And use ImmutableTransaction on more places and other harmonizations.