[Dataset scripts] add all datasets scripts
As mentioned, we can have the canonical datasets in the master. For now I also want to include all the data as present on S3 to make the synchronization easier when uploading new datastes. 

@mariamabarham @lhoestq @thomwolf - what do you think? 

If this is ok for you, I can sync up the master with the `add_dataset` branch: https://github.com/huggingface/nlp/pull/37 so that master is up to date. 