rnn_echo.py: an exercise of "echo", refering to https://yq.aliyun.com/articles/177529?t=t1 .
rnn_mnist.py: an exercise of "MNIST", refering to http://blog.csdn.net/qq_16949707/article/details/53258498 . Used LSTM.
In mnist.py, time is in x-dirction, i.e., from left to right, which is different from the reference above.
