'dynamically add a marker object to the node.
``marker`` can be a string or pytest.mark.* instance.'

def add_marker(self, marker):

    from _pytest.mark import MarkDecorator, MARK_GEN
    if isinstance(marker, py.builtin._basestring):
        marker = getattr(MARK_GEN, marker)
    elif (not isinstance(marker, MarkDecorator)):
        raise ValueError('is    not    a    string    or    pytest.mark.*    Marker')
    self.keywords[marker.name] = marker
