'List the n most common elements and their counts from the most
common to the least.  If n is None, then list all element counts.
>>> Counter(\'abracadabra\').most_common(3)
[(\'a\', 5), (\'r\', 2), (\'b\', 2)]'

def most_common(self, n=None):

    if (n is None):
        return sorted(iteritems(self), key=itemgetter(1), reverse=True)
    return nlargest(n, iteritems(self), key=itemgetter(1))
