[WIP] MultiSafepay implementation

Opening the MultiSafepay pull request early as mentioned in #83.
Obvious things missing: 

* more tests (mock responses)
* handling of the other statuses that might be returned by the gateway in ```CompletePurchaseResponse```
* setting of return, cancel, etc... URLs, for now only required fields are sent in ```PurchaseRequest```

The docs for MultiSafepay are here: https://www.multisafepay.com/downloads/handleidingen/Handleiding_connect(ENG).pdf