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.



I didn't understand the adyen part of your comment... but about the first part, if you want to assume it's a dict, you must not assume it's a dict of list, so use `value[0]` only if value is a list