Add support for german license plates

Since they have a special structure, I needed to remove the
normalization of the license plate string.
Add info about german license plates to README
Merge branch 'feature/german_license_plate_format' of git://github.com/evopark/license_plate_validator into evopark-feature/german_license_plate_format
Merge branch 'evopark-feature/german_license_plate_format'