Allow chaining of Product's custom querysets

This aligns the implementation of Oscar specific QuerySet Methods with
the implementation in current django core[1].
While this is not DRY, it does deliver on chainability and can be seen
as preparation to take advantage of the improvements coming to this part
of django in 1.7 [2]

The original motivation for this was that using a custom QuerySet while
trying not to copy code from oscar felt too hard.

[1] https://github.com/django/django/blob/1.6.2/django/db/models/manager.py#L123
[2] https://docs.djangoproject.com/en/dev/releases/1.7/#calling-custom-queryset-methods-from-the-manager

Fixes #1278.

Deprecation comments added by @maikhoepfel

oscar/apps/catalogue/managers.py

 from django.db import models
 
 
-class ProductManager(models.Manager):
+class ProductQuerySet(models.query.QuerySet):
 
     def base_queryset(self):
         """
-        Return ``QuerySet`` with related content pre-loaded.
+        Applies select_related and prefetch_related for commonly related
+        models to save on queries
         """
-        return self.get_query_set().select_related('product_class')\
+        return self.select_related('product_class')\
             .prefetch_related('variants',
                               'product_options',
                               'product_class__options',
                               'stockrecords',
                               'images',
-                              ).all()
+                              )
+
+    def browsable(self):
+        """
+        Excludes non-canonical products.
+        """
+        return self.filter(parent=None)
+
+
+class ProductManager(models.Manager):
+    """
+    Uses ProductQuerySet and proxies its methods to allow chaining
+
+    Once Django 1.7 lands, this class can probably be removed:
+    https://docs.djangoproject.com/en/dev/releases/1.7/#calling-custom-queryset-methods-from-the-manager  #n oqa
+    """
+
+    def get_queryset(self):
+        return ProductQuerySet(self.model, using=self._db)
+
+    def browsable(self):
+        return self.get_queryset().browsable()
+
+    def base_queryset(self):
+        return self.get_queryset().base_queryset()
 
 
 class BrowsableProductManager(ProductManager):
     """
     Excludes non-canonical products
+
+    Could be deprecated after Oscar 0.7 is released
     """
 
-    def get_query_set(self):
-        return super(BrowsableProductManager, self).get_query_set().filter(
-            parent=None)
+    def get_queryset(self):
+        return super(BrowsableProductManager, self).get_queryset().browsable()