Use Oscar dynamic loading with get_class

One of the main concern about django-oscar-adyen is that one may want
to override parts of the application. For example, how the merchantData
are used, or what to do to record a transaction.

Since there is no one-size-fit-all, I reuse here a very convenient tool
from django-oscar: `oscar.core.loading.get_class`. This tool allows us
to provide a base interface, that can be used almost out of the box,
and make possible anyone to override parts of it.

This is only the first step to a real modular django-oscar plugin, but
that's a good first step.

adyen/facade.py

@@ -1,13 +1,19 @@
 # -*- coding: utf-8 -*-
 
-import iptools
 import logging
 
+import iptools
 from django.http import HttpResponse
+from oscar.core.loading import get_class
 
-from .gateway import Constants, Gateway, PaymentNotification, PaymentRedirection
-from .models import AdyenTransaction
 from .config import get_config
+from .models import AdyenTransaction
+
+Constants = get_class('adyen.gateway', 'Constants')
+Gateway = get_class('adyen.gateway', 'Gateway')
+PaymentNotification = get_class('adyen.gateway', 'PaymentNotification')
+PaymentRedirection = get_class('adyen.gateway', 'PaymentRedirection')
+
 
 logger = logging.getLogger('adyen')
 

adyen/models.py

@@ -1,10 +1,11 @@
 # -*- coding: utf-8 -*-
 
-from django.db import models
 from django.conf import settings
+from django.db import models
 from django.utils import timezone
+from oscar.core.loading import get_class
 
-from .gateway import Constants
+Constants = get_class('adyen.gateway', 'Constants')
 
 
 class AdyenTransaction(models.Model):

adyen/scaffold.py

@@ -1,9 +1,12 @@
 from django.utils import timezone
+from oscar.core.loading import get_class
 
-from .facade import Facade
-from .gateway import Constants, MissingFieldException
 from .config import get_config
 
+Constants = get_class('adyen.gateway', 'Constants')
+Facade = get_class('adyen.facade', 'Facade')
+MissingFieldException = get_class('adyen.gateway', 'MissingFieldException')
+
 
 class Scaffold: