* Implemented faster attributes save

* renamed for clarity

…ct to avoid transferring (#331)

attributesvalues from parent to child.

Use the same database as the passed-in manager

* The image and file fields where nolonger working.

This PR restores that functionality.

* nergens voor

When an image product attribute is `null`, the product API would throw the
following exception:

```
ValueError: The 'value_image' attribute has no file associated with it.

  […]

  File "rest_framework/serializers.py", line 522, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "rest_framework/serializers.py", line 686, in to_representation
    return [
  File "rest_framework/serializers.py", line 687, in <listcomp>
    self.child.to_representation(item) for item in iterable
  File "rest_framework/serializers.py", line 522, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "oscarapi/serializers/fields.py", line 227, in to_representation
    return value.value.url
  File "django/db/models/fields/files.py", line 66, in url
    self._require_file()
  File "django/db/models/fields/files.py", line 41, in _require_file
    raise ValueError(
```

This patch fixes this by checking if the image field has a value before trying
to generate a URL.

Fixes handling of null product image attributes

Add category bulk admin api

* Doe some updating and add test

* refrhes_from_db only path

* Add transaction that rollsback on error

* update versions