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.



Empirically it worked just fine on all my test runs, but then again I'm not sure if the execution order of a test suite is guaranteed to be deterministic, so it may just have ran last and caused this side effect without me noticing...
What do you think?
Should I duplicate the <code>override_settings</code> clause over all the test methods just to be sure?