Refs #654 -- API: Status operations on orders resource (#640)

* API: Write operations on orders resource

* Add order API endpoint /extend/

src/pretix/settings.py

@@ -252,6 +252,7 @@ REST_FRAMEWORK = {
     'DEFAULT_RENDERER_CLASSES': (
         'rest_framework.renderers.JSONRenderer',
     ),
+    'EXCEPTION_HANDLER': 'pretix.api.exception.custom_exception_handler',
     'UNICODE_JSON': False
 }