Attribute 'score' in Product model breaks the use of haystacks simple backend

I just came across an issue where haystack's simple backend raises an exception:

```TypeError: "__init__() got multiple values for keyword argument 'score' ```

This is caused by Oscar's ``Product`` model and its attribute ``score``. A corresponding issue is mentioned in [streeter's comment](https://github.com/toastdriven/django-haystack/issues/482#issuecomment-7473328).

I am aware that this might not be something that will be fixed as the only solution in Oscar that I can think of is changing the attribute ``score`` to something else. I thought, I'd put it in here though, in case some one comes across the same issue. 

The easiest solution seems to be to use  Mplsbeb/whoosh instead of the simple backend or patching the simple backend by removing the ``score`` attribute before passing it into the constructor of the result class in [line 74](https://github.com/toastdriven/django-haystack/blob/master/haystack/backends/simple_backend.py#L74).