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 think adding `override_settings` with only `ADYEN_ACTION_URL` on this method only should work (the settings will be restored when exiting the method)

Also, it will be easier in 1.7 https://code.djangoproject.com/ticket/20032