Added support for discounts and buses, rewrote collmex interface

Signed-off-by: Malte Sussdorff <malte.sussdorff@cognovis.com>
Ability to deal with helper rebate

Signed-off-by: Malte Sussdorff <malte.sussdorff@cognovis.com>
new invoices

Signed-off-by: Malte Sussdorff <malte.sussdorff@cognovis.com>