Assert n elements exist matching the given selector.

def count_elements_exactly_by_selector(self, number, selector):
    """
    Assert n elements exist matching the given selector.
    """
    elems = find_elements_by_jquery(world.browser, selector)
    number = int(number)
    if len(elems) != number:
        raise AssertionError("Expected {} elements, found {}".format(
            number, len(elems)))