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.



ok, but as you said " I think considering it as a dict is a good approach", i thought i may be a simple dict not related to `parse_qs`, because i didn't check the line above, my bad