Add i18n-js to OpenProject

# Why client side I18n?

Client-side I18n is needed in a multitude of cases. At the moment, the two UI widgets in OpenProject, Calendar and Wiki Toolbar, use two separate I18n solutions. Every plugin, that is trying to do client-side I18n, will need to ship a third, fourth and fifth solution.

By introducing i18n-js into the OpenProject core, we can provide a single solution for future enhancements of the core and a generic solution for plugin authors.

# Why i18n-js?

* The plugin seems to be rather popular (Starred: > 500, Forked: > 100 atm). 
* It claims to support Rails 3 (not tested by me). 
* Its working principals are simple enough to take over development or replace it with something else, if need be.

This pull request is based on #[52](https://github.com/finnlabs/chiliproject/pull/52) in the finnlabs/chiliproject repository.

# Impact on existing users

The install guide needs to be changed. The `public/javascripts/i18n` directory needs to writable, just like the `public/plugin_assets`.

# Usage

I18n keys in the `js` scope will be available on the client side.

    en:
      js:
        hello: Hello, World!

Especially for plugin authors, also js scopes in deeper nestings will be available

    en:
      my_plugin:
        js:
          hello: Hello from plugin!

In your JS code, use the following commands:

    I18n.t('js.hello'); // => "Hello, World!"
    I18n.t('js.my_plugin.hello'); // => "Hello from plugin!"