Add test confirming all locales have all keys.

Something that has bit our team a few times: we add a new key to the
default locale file, but forget to add it to the others.

This adds an test that runs in the consuming app, that confirms all
locale files contain the keys from the `defaultLocale`.
Merge pull request #17 from rwjblue/add-test-confirming-all-locales-have-all-keys

Add test confirming all locales have all keys.