Add MinimumImpl and remove BroadcastQuaternary
Add MinimumImpl and remove BroadcastQuaternary

chainer/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,okuta/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,wkentaro/chainer,pfnet/chainer,tkerola/chainer,chainer/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,keisuke-umezawa/chainer