Translate Preface, Intro, and Fin.

Yohei Yasukawa (@yasulab), Johnny Mukai (@johnnymugs), Shozo Hatta, and Junichi Chiba start translating the following pages in RHG. We'll commit and push our updates to this branch.

- [Preface](http://loveruby.net/ja/rhg/book/preface.html)
- [Intro](http://loveruby.net/ja/rhg/book/intro.html)
- [Final Chapter](http://loveruby.net/ja/rhg/book/fin.html)