Seq2seq tutorial

[jumerckx]

As suggested by @MikeInnes in #69, I'm making a pull request with a seq2seq tutorial in literate.jl format.

I'm hoping someone could have a look at this model since performance (translation quality) isn't very good.

Thanks

[MikeInnes]

Would be nice to figure out the training issues so that this can get more interesting results before we merge. Also, you shouldn't need to commit the ipynb file here since we have a conversion script that will generate it from the latest .jl version.

[jumerckx]

Thanks for the feedback, I've removed the notebook version.
As for the translation quality, I'm afraid I'm unable to fix this, although I'm pretty certain I'm making a silly mistake somewhere.