JSON script
The tests for the Wikipedia dataset do not pass anymore with the error:
```
To be able to use this dataset, you need to install the following dependencies ['mwparserfromhell'] using 'pip install mwparserfromhell' for instance'
``` This was an issue on master. You can just rebase from master. Perfect! Indeed, it worked^^ Thanks @lhoestq  Currently the dummy_data tests are always green because in a PR the dataset is not yet synchronized with aws. This PR fixes this: https://github.com/huggingface/nlp/pull/140 . 

Could you test `json` locally or wait until the PR: https://github.com/huggingface/nlp/pull/140 is merged ? :-)  Ok, I will wait #140 to be merged and then rebase :) 
Add a JSONS script to read JSON datasets from files.