added ZippedImageDataset and MultiZippedImageDataset to document
added ZippedImageDataset and MultiZippedImageDataset to document

pfnet/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,hvy/chainer,ktnyt/chainer,ktnyt/chainer,keisuke-umezawa/chainer,wkentaro/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,anaruse/chainer,ktnyt/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,jnishi/chainer,chainer/chainer,ronekko/chainer,jnishi/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,wkentaro/chainer,jnishi/chainer,hvy/chainer,rezoo/chainer,okuta/chainer,niboshi/chainer,jnishi/chainer,niboshi/chainer,tkerola/chainer,ktnyt/chainer,chainer/chainer