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