Add period to all full sentences in the translation file.
Raise MissingTranslationData in tests only, now works.

Removed the engine’s initializer since it’s not the engine’s business
how to handle missing translations. Instead, the user of the gem should 
configure his Rails application accordingly.

Added `config.action_view.raise_on_missing_translations` (new in recent Rails 4 versions) to our test helpers and demo Rails app, so we can can still see any problems.
Add missing translations for demo app.
Fix more missing translations.

After getting the exception on missing translations to work, many more
missing translations showed up. This fixes them.

I believe I’m passing the string „typus.buttons.xy“ through the application 
more than necessary, the translation of the action names can probably be done lower down in the stack, later.