seq2seq learning

• this is motivated by TensorFlow's tutorial: https://www.tensorflow.org/tutorials/seq2seq
• but the model structure is more like that stated in this paper: https://arxiv.org/abs/1409.0473
• I wrote these codes to help me better understanding the details of the seq2seq model
• WARNING: these are very rush codes,
• Although I've got reasonable results (in terms of BLEU) on the 12 million en-fr dataset