api shouldn't know about order and token

ping @makasim 

This connects with one of the comment you made in payum repo
```
this logic has to be moved out of api class, ideally to a FillPaymentDetailsAction. The api class provide a low level access it is not aware of any models , tokens security and so on.
```

let me know if it's better now. 

question: could we access api constants from fillOrderDetailsAction and maybe get ride of the completePayment Method?

Also please note i pushed a changed to the master branch related to payment urls. You were redirected to the wrong url in your yesterday tests. Was my fault. Should be ok now. 
mmm, what i want here is given the order currency get the redsys code for it. so if i have 'EUR' as order->getCurrency() 
i call the api to return the value for the currencies['EUR'] wich is 978 in the case, the one the bank expects. Seems correct to me... or are you referring to the name of the function?