http://www.elasticsearch.org/guide/reference/query-dsl/geo-bounding-box-filter.html

        > bounds = ElasticFilter().geo_bounding_box('pin.location', [40.73, -74.1], [40.717, -73.99])
        > bounds = ElasticFilter().geo_bounding_box('pin.location', dict(lat=40.73, lon=-74.1), dict(lat=40.717, lon=-73.99))
        > bounds = ElasticFilter().geo_bounding_box('pin.location', "40.73, -74.1", "40.717, -73.99")
        And geohash
        > bounds = ElasticFilter().geo_bounding_box('pin.location', "drm3btev3e86", "drm3btev3e86")

def geo_bounding_box(cls, field, top_left, bottom_right):
        '''
        http://www.elasticsearch.org/guide/reference/query-dsl/geo-bounding-box-filter.html

        > bounds = ElasticFilter().geo_bounding_box('pin.location', [40.73, -74.1], [40.717, -73.99])
        > bounds = ElasticFilter().geo_bounding_box('pin.location', dict(lat=40.73, lon=-74.1), dict(lat=40.717, lon=-73.99))
        > bounds = ElasticFilter().geo_bounding_box('pin.location', "40.73, -74.1", "40.717, -73.99")
        And geohash
        > bounds = ElasticFilter().geo_bounding_box('pin.location', "drm3btev3e86", "drm3btev3e86")

        '''
        return cls(geo_bounding_box={field: {'top_left': top_left, 'bottom_right': bottom_right}})