Support for dynamic query operator

Right now, there is `HAYSTACK_DEFAULT_OPERATOR`, which can be set in settings, and which is used for instance to generate proper schema.xml for SOLR.

That settings so far cannot be changed per query.

I have prepared simple functionality to SearchQuerySet, which allow change default behaviour

```` Python
    if self.get_args['wide'] == True:
        self.searchqueryset = self.searchqueryset.query_operator("OR")
````

I am sure, that this code works well for SOLR. I believe that this should also works for elasticsearch thanks to my changes, however I had no oportunity to test this for ES.