Import signal receivers in __init__.py (not models.py)

This is to avoid circular import issues as most receivers.py modules
also import models.  It's not entirely clear what circumstances are
required to get the circular import - it seems to be when using
django-debug-toolbar>=1.0, which changes the way the URLs are
monkey-patched.

This changes adopts the recommended practice for importing signal
receivers:

- https://groups.google.com/forum/#!msg/django-developers/PReHoxwwG_c/M-asj72ks7YJ
- http://stackoverflow.com/a/14192257/25517

Fixes #1125 (I think)

oscar/apps/analytics/__init__.py

+from . import receivers  # noqa

oscar/apps/analytics/models.py

 
 class UserSearch(AbstractUserSearch):
     pass
-
-
-# Import receiver functions
-from oscar.apps.analytics.receivers import *  # noqa

oscar/apps/catalogue/__init__.py

+from . import receivers  # noqa

oscar/apps/catalogue/models.py

 
 class ProductImage(AbstractProductImage):
     pass
-
-from .receivers import *  # noqa

oscar/apps/catalogue/receivers.py

             except ThumbnailError:
                 pass
 
+
 # connect for all models with ImageFields - add as needed
 models_with_images = [ProductImage, Category]
 for sender in models_with_images:

oscar/apps/customer/__init__.py

+from oscar.apps.customer.history import *  # noqa
+from oscar.apps.customer.alerts.receivers import *  # noqa

oscar/apps/customer/models.py

 
 class ProductAlert(abstract_models.AbstractProductAlert):
     pass
-
-
-from oscar.apps.customer.history import *  # noqa
-from oscar.apps.customer.alerts.receivers import *  # noqa

oscar/apps/offer/receivers.py

 from django.db.models import get_model
 
 from oscar.apps.basket.abstract_models import AbstractBasket
+
 Voucher = get_model('voucher', 'Voucher')
 OrderDiscount = get_model('order', 'OrderDiscount')
 ConditionalOffer = get_model('offer', 'ConditionalOffer')

oscar/apps/partner/__init__.py

+from oscar.apps.partner.receivers import *  # noqa

oscar/apps/partner/models.py

 
 class StockAlert(AbstractStockAlert):
     pass
-
-
-from oscar.apps.partner.receivers import *  # noqa

oscar/apps/shipping/models.py

 
     @property
     def charge_incl_tax(self):
-        # Note, when weighing the basket, we don't check whether the item
+        # Note, when weighing the basket, we don't check whether the item
         # requires shipping or not.  It is assumed that if something has a
         # weight, then it requires shipping.
         scales = Scales(attribute_code=self.weight_attribute,