Add support for SQ in SearchQuerySet.narrow()

Example:
```py
AND = lambda x: SQ(reduce(operator.__and__, x))
OR = lambda x: SQ(reduce(operator.__or__, x))

categories = ['foo', 'bar', 'hello', 'world']

q = OR([SQ(category=cat) for cat in categories])

sqs = SearchQuerySet().model(Product).narrow(q).facet('color')
```