adding snow_simplified_japanese_corpus
Adding simplified Japanese corpus "SNOW T15" and "SNOW T23".
They contain original Japanese, simplified Japanese, and original English (the original text is gotten from en-ja translation corpus). Hence, it can be used not only for Japanese simplification but also for en-ja translation.

- http://www.jnlp.org/SNOW/T15
- http://www.jnlp.org/SNOW/T23
merging since the CI is fixed on master Thank you for the updates and merging!