Internationalization Setup

[Pivotal Tracker](https://www.pivotaltracker.com/story/show/67496894)

* Extended `ActionView::Helpers::TranslationHelper` to include indicators for translated text in all environments except `production`
* Add locale to URL parameters
* Add a locale switch in the header
* Basic translation skeletons added to `config/locales/` with a simple file structure
* Add `binding_of_caller` gem for use with `better_errors` (should have been added the first time)

**Note** ActiveRecord does not recognise default translations for some reason, despite following the documentation. A bug has been set up -> [here](https://www.pivotaltracker.com/story/show/70226658)