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 failure handling for SellerExchangeActor