feature: multiple parcel shipment

* add multiple parcel shipment
* add new service called ServiceGroupDHLPaket
* fix spec requires
* update doc
feature: multiple parcel shipment

* add deprication warning for single shipment construction
feature: add dhl domestic express (DD shipments)

* add new services: HigherInsuranceService, DhlExpressService
* rename group_dhl_paket_service to dhl multipack service
* fix tests
* update README
Update README.md
Merge pull request #8 from webit-de/master

Add multiple parcel shipment and DHL Domestic Express