Format the mean score +/- std error for params

def display_scores(params, scores, append_star=False):
    """Format the mean score +/- std error for params"""
    params = ", ".join("{0}={1}".format(k, v)
                      for k, v in params.items())
    line = "{0}:\t\t{1:.3f} (+/-{2:.3f})".format(
        params, np.mean(scores), sem(scores))
    if append_star:
        line += " *"
    return line