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.



According to [the urllib.parse.parse_qs documentation](https://docs.python.org/3.3/library/urllib.parse.html?highlight=urllib%20parse_qs#urllib.parse.parse_qs):
> The dictionary keys are the unique query variable names and the values are lists of values for each name.

So I think it's fine. :smiley: