"Competitive Example" (2.5): Sequence-to-sequence

[neubig]

Part of: #1284

We have an example of a sequence-to-sequence model:

https://github.com/clab/dynet/tree/master/examples

We should make sure that it is obvious how to download training data, and that the model is at least as good as the PyTorch example:

http://pytorch.org/tutorials/intermediate/seq2seq_translation_tutorial.html

[gpengzhi]

#1391