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).

Can't we use `type Paid[C <: FiatCurrency] = Payment[C]` for now?