Add spanish billion words
The tests failed because of ```RemoteDatasetTest``` so I tried ```git rebase``` and messed everything up. I've made a new clean PR (#1347).
Add an unannotated corpus of the Spanish language of nearly 1.5 billion words, compiled from different resources from the web.

The dataset needs 10 GB (download: 1.89 GiB, generated: 8.34 GiB, post-processed: Unknown size, total: 10.22 GiB), the test using dummy data pass but my laptop isn't able to run it on the real data (I left it running for over 8 hours and it didn't finish).