Feature/initial

This is the first iteration of the Adyen payment backend for Oscar.
It still needs documentation, which I am going to improve during the day, but I figure it's worth starting the review ASAP.

As for the other backends, it follows the <code>scaffold —> facade —> gateway</code> pattern.

Some of it might seem overengineered right now but this is to prepare a future, fuller implementation.



Added a base <code>BaseInteraction</code> class, common ancestor of both <code>BaseRequest</code> and <code>BaseResponse</code> to refactor this code.
Thanks for the suggestion. :smile: 

:heavy_check_mark: