Add domain model for the exchange protocol
Move main window to a new package
Introduce the concept of application view
Introduce a view selector
Introduce CSS for the wizard
Move multisig deposit creation to TransactionProcessor
Minimize the impact of Transaction being mutable
Merge pull request #178 from sortega/task/41_abstract

Introduce untested domain model for the exchange protocol
Require at least one view in ApplicationScene
Merge pull request #179 from sortega/task/54_main_view

Introduce views and CSS into the UI
Turn `PaymentProcessor` into an actor

`PaymentProcessor` trait is aimed to be used in an asynchronous way by
producing `Future` objects. Since we already have Akka to deal with
async IO, it is convenient to consider the payment processors as
another actor.

This change turns `PaymentProcessor` trait into a hierarchy of
actors. The same functionality is implemented in a message-passing
approach.

This addresses [task 52](http://redmine.coinffeine.com/issues/52).
Add ExchangeSupervisorActor
Rename exchange-related actor classes
Include a `FindPaymentFailed` in `PaymentProcessor`

This kind of message was missing. It is useful to report an error
ocurred while finding a payment.
Merge pull request #181 from edrevo/add-exchange-supervisor

Add ExchangeSupervisorActor
Merge pull request #180 from apoloval/issues/52

Turn `PaymentProcessor` into an actor
Fix build break
Merge pull request #185 from edrevo/fix-build-break

Fix build break
Add test for handshake start

Also, fix issues to make it pass
Refine and test the exchange model

Refine and test the exchange model up to the handshake step