I18n helper

- [x] wrapped the `I18n.lookup` function to inject the "js" namespace so that we can use *all* the functions from the `I18n` library instead of *only* the `translate` one.
- [x] renamed **all** the calls to `Em.String.i18n` to the shorter `I18n.t`

**Note:** couldn't find a way to modify directly the `arguments` object without jshint blowing up. Had to fool it...