http://deeplearning.stanford.edu/wiki/index.php/UFLDL%E6%95%99%E7%A8%8B
http://neuralnetworksanddeeplearning.com/chap2.html
http://karpathy.github.io/neuralnets/
https://www.zhihu.com/question/28385679
https://chrischoy.github.io/research/making-caffe-layer/