Implemented localization throughout Pakket + added helper functions

* Now using the translation features thoroughly across all default templates. 
e.g. `{{ t('hello') }}` => Hello world.

* The error pages are now using the asset pipeline to serve their CSS as well.
Splitting the helpers into a separate repository seems to be a bit overkill. At the moment they're located in `/app/helpers` and this little foreach loop autoloads them. Perhaps there's another approach to autoload them?