[IMP] Add class CarrierAccount, so the specifics carriers modules use all the same base inheriting this one
[IMP] Change get_label_sender_address so it gets delivery contact instead of parent
[FIX] _get_label_sender_address method + pep8
[FIX] Change class CarrierAccount in its own python file
Merge pull request #17 from florian-dacosta/7.0

7.0 Add class CarrierAccount, so the specifics carriers modules use all the same base