Support translations

### Changes
This pull-request support translations. I've added Japanese translation.
A visitor can read the translated articles by selecting a language from the navigation at the bottom of pages.

![screenshot1](http://gyazo.com/9f35865331139212a580d6e1b32bedd4.png) &nbsp; ![screenshot2](http://gyazo.com/1e23b693e2a5440a47b757cad7f55178.png)

Paths of pages are followings: 
- `/`: The home page in English.
- `/:factor`: Articles in English, e.g. `/codebase`.
- `/ja/`: The home page in Japanese.
- `/ja/:factor`: Articles in Japanese, e.g. `/ja/codebase`.

### Motivation

Last August, I have translated 12factor into Japanese and published at [http://twelve-factor-ja.herokuapp.com/](http://twelve-factor-ja.herokuapp.com/).
A few thousands of people in Japan have read the translation up to now.

If the official site provide translations, a visitor can easily find articles translated into his/her native language.
Then, more and more people can know 12factor's idea, use as a shared vocabulary and discuss on it.

There seem to be some people who are interested in translations:
- [Any plans on supporting translations · Issue #14 · adamwiggins/12factor](https://github.com/adamwiggins/12factor/issues/14)
- [liangshan/12factor](https://github.com/liangshan/12factor)
- [anjuke/12factor](https://github.com/anjuke/12factor)

It's wonderful if the pull-request is merged and translations in other languages are available.