Translate Intro

In the same way as [translating Preface](https://github.com/ruby-hacking-guide/ruby-hacking-guide.github.com/pull/5),  Junichi Chiba, Shozo Hatta, Johnny Mukai (@johnnymugs), and I (@yasulab) are translating Intro into English.

We focus on roughly translating from Japanese into English. We're really glad if someone help reviewing and correcting our translations to merge.