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.



Well spotted!
Do you think it's safe to assume that if the <code>HTTP_X_FORWARDED_FOR</code> header is present, it will *always* supersede <code>REMOTE_ADDR</code>?
My first thought was to add a setting like <code>USE_WEB_PROXY</code> to decide what to do, but I think it would be more elegant to simply make the previous assumption.