zh-tw subdomain

Hi,

We recently run many workshops using your RailsBridge document. It's pretty good for newbies. Thanks for your great work!

However the URL of document is hard to remember. Currently it's http://railsbridge-docs-zh-tw.herokuapp.com/ with a short cut URL http://bit.ly/railsbridge-tw

It would be great if the URL becomes http://zh-tw.railsbridge.org so it is easier to remember and people who read Chinese can link to our version from your website.

By merging this pull request, the nav bar will be look like this, which you can see in our current website:

![screen shot 2014-06-25 at 09 51 42](https://cloud.githubusercontent.com/assets/10737/3380614/4b575d7c-fc0b-11e3-88c9-0a703768c23d.png)

However, in addition to merging PR, to serve an aliased domain name, there are some DNS setups that should be configured (as mentioned in #294). Would you do this for us?

And by the way, I see in [`app.js/locale`](https://github.com/alexch/railsbridge-docs/blob/master/app.rb#L73) there is a comment saying that locale-by-subdomain supports only 2-digits language name. We're using 5 digits (`zh-tw`), but there is no locale-switching issue on our Heroku deployment, because we've forcibly set locale to zh-tw when running up the server. There might be another issues in the future but not urgent.

Thanks again!
Would be nice if this list used `::Installfest::AVAILABLE_LOCALES` instead. 

(Also if `Installfest` were renamed `RailsbridgeDocs` :-))