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
Updated gem version to 0.4. Added multiple parcel shipment and domestic express support
bugfix: multipack service is not allowed for EPX

Multi parcel shipment by DHL Express is standard,
so we only have to add multiple shipment items.
Merge pull request #9 from webit-de/master

bugfix: multipack service is not allowed for EPX
Upped minor version
Merge remote-tracking branch 'teameurope/master'