参考：
Understanding LSTM Networks
(Karpathy blog )The Unreasonable Effectiveness of Recurrent Neural Networks
(课程)CS224d: Deep Learning for Natural Language Processing
(代码解析)Anyone Can Learn To Code an LSTM-RNN in Python
(代码解析)深度学习笔记(四)：循环神经网络的概念，结构和代码注释
(论文)Recurrent neural network based language model
(论文)Gated Feedback Recurrent Neural Networks
1、Long-Term Dependencies
2、梯度消失和爆炸