Added Informal German translation.

* Added Informal German translation.
  I've also tried to make it a bit more gender-neutral.
  For example, I've replaced the word "Berater" (German
  male word for operator) with the word Operator (still
  a male word, but less confusing if a female operator
  is on the other line)
* Some minor corrections in the de_DE translation

Since Transifex is not supporting informal translations, I'm making a pull request for this update. 
Further info: http://www.cotonti.com/forums?m=posts&q=7367&d=2

Since there are other languages which could also be informal (Dutch, German, etc), I suggest to create a new Transifex Project for informal versions.