Add spanish billion words corpus
Add an unannotated Spanish corpus of nearly 1.5 billion words, compiled from different resources from the web.
Thank you for your feedback! I've reduced the dummy data size to 2KB.

I had to rebase to fix `RemoteDatasetTest` fails, sorry about the 80 commits. 
I could create a new clean PR if you prefer. I have seen that in similar cases you have suggested to other contributors to create another branch and another PR, so I will do that. Yes thank you ! The new PR is #1476 :)