Implements Multiclass hinge loss

Implements multiclass hinge loss. Fixes #3451 
Sorry for misleading you. I had meant something like this (untested).

    hinge_loss = np.ones(y_true.shape[0])
    for i in np.unique(y_bin.classes_):
        pos_index = y_bin == i
        hinge_loss[pos_index] -= pred[pos_index][i]
        hinge_loss[~pos_index] += pred[~pos_index][i]
    return np.mean(hinge_loss)