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.



You may very well be right there.
However, we are at the <code>gateway</code> level, which is supposed to be independent of Django, so I think considering it as a <code>dict</code> is a good approach — especially since we know Adyen will never give us back a query string with array-like parameters, which are afaict one of the main reasons <code>OrderedDict</code>s exist in the first place.