Extract taxonomy jstree menu to a new file

Fixes #2953
Translate everything within core
Implement I18n testing

This allows us to check for:
1) Keys used during the test which have no translations
2) Keys that aren't used during the tests at all.

Run the tests with 'CHECK_TRANSLATIONS=1 bundle exec rspec spec' and watch the
fireworks.
BigDecimal#inspect objects now look better

Thanks to @mipearson for this tip: https://gist.github.com/mipearson/5410134
Support multiple translation keys in faux-Spree.t
Move I18n testing stuff into spree/testing_support
Add translation checking to frontend
[backend] convert over spree/admin/shared/_address_form.html.erb + refactor it
Use Spree.t within frontend views
Correctly search for keys in testing_support/i18n
exit with status code 1 if translations are missing / unused
Remove defunct translation keys, etc.

* Remove everything from `config/locales/en.yml` that couldn't be found with ack commands in `missing_translations.rb`

* Add translation for country_names.US

* Additional translation tweaks
Make translations in backend controllers, helpers, and views now use Spree.t
Require correct file in i18n_spec
Fix translations across api, core and frontend
Add missing translations for mail_method_settings, quantity, stock_locations and stock_movements
Namespace dash translations
Re-add shipping translation
Translate descriptions for shipping calculators using Spree.t
Move activerecord translations out of spree namespace