Add spanish billion words corpus
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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 :)
Add an unannotated Spanish corpus of nearly 1.5 billion words, compiled from different resources from the web.
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