https://blog.csdn.net/abcjennifer/article/details/7748833
https://blog.csdn.net/zouxy09/article/details/24971995
https://blog.csdn.net/abcjennifer/article/details/7721834
http://deeplearning.stanford.edu/wiki/index.php/%E7%A8%80%E7%96%8F%E7%BC%96%E7%A0%81
http://deeplearning.stanford.edu/wiki/index.php/Exercise:Sparse_Coding