[wip] german translation updates

This PR is based on #23 and there are still some open tasks dependent on issues like #103 and #104.

The translation is not yet complete (some terms and are not quite easy to translate), so please do not merge it unasked. I just wanted to start discussion for those translations.
I also switched some links to the german SensioLabs part so german readers will directly hit the correct target.
I'll give feedback when the PR is complete.

Side note: At several places `SensioLabs` is still written the old way `Sensio Labs`.