Add Germany national holidays

I added Germany's country-wide holidays.

There are also state holidays. Not sure how to add those. Should I make a new class for each one? Are there any naming conventions? We have some very bulky state names, like "Baden-Württemberg". I'd suggest using abbreviations.
I'd recommend to use [the Boxing Day flag and the Boxing Day label](https://github.com/novapost/workalendar/blob/master/workalendar/core.py#L209-L210) to add this religious holiday.