Replace Typus::I18n#t with I18n#t or just #t
Add translation file for English
Prevent Typus:I18n#t from being used right now (still work in progress).
Typus now does I18n the default Rails way, i.e.:

- to translate, I18n#t is called directly, not Typus::I18n#t.
- extracted all English texts from the code to a YAML translation file.
- created some hierarchy in the resource keys.
- adapted the German translation file to the changes; the others that we understand will follow soon.
- added translation file for dummy app (only contains some model fields and attribute values)
Disable test until Typus::I18n is replaced.
Translation file for model attributes in the dummy/demo app.
Initializer for i18n. 

For missing translations it will do locale fallbacks in production,
all other environments will raise an exception (so problems can be fixed quickly).
Fix all i18n-related test failures.

I had missed a few translations, and probably there are few more; need to test manually.