German translation added, closes #155

Things left to do: 
  - test the game and maybe edit some CSS (can I test my local changes?)
  - convert the po file to a js file. Script did not work. ImportError: No module named polib.

    That is odd, because I used poedit to translate. OS: ubuntu 14.04
  - get some insights for the 26 fuzzy translations. I will add a list with questions.