Return a JSON string export of the pages in queryset.

def pages_to_json(queryset):
    """
    Return a JSON string export of the pages in queryset.
    """
    # selection may be in the wrong order, and order matters
    queryset = queryset.order_by('tree_id', 'lft')
    return simplejson.dumps(
        {JSON_PAGE_EXPORT_NAME: JSON_PAGE_EXPORT_VERSION,
            'pages': [page.dump_json_data() for page in queryset]},
        indent=JSON_PAGE_EXPORT_INDENT, sort_keys=True)