I18n

Hi,

We're going to host a workshop in Taiwan, which uses RailsBridge materials. Obviously it should be translated into Traditional Chinese, the language we use in Taiwan.

During translation I found that there are some words that cannot be translated until I18n is applied. So I implemented this.

After this PR I will send another PR which adds zh-tw locale into app.rb and the flag of Taiwan at the top-right corner.

By the way, we've done translating "Installfest" and "Intro to Rails" sections. Here is the URL: http://railsbridge-docs-zh-tw.herokuapp.com/docs/

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**Update:** further details:

For common words, like "Next Step", "Step X", they're going to be localized in `locales/<locale>.yml` file.

For page names, currently English is automatically generated from file name, like what it does currently. For other languages, page names should be translated in `locales/pages.<locale>.yml` file. If the translation is missing, it will take the auto-generate strategy, which fallbacks to English. There is a `locales/pages.en.yml` file for those who're going to localize page names.