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/