Merge pull request #183 from benzittlau/extend-payment-methods

Adding additional support for payment methods.
Merge pull request #184 from benzittlau/add-bill-line-item-helpers

Add line item helpers to bill line item model.
Merge pull request #185 from benzittlau/total_amount_consistency

Make the API for total amounts consistent
Merge pull request #186 from benzittlau/add-bill-payment-deletion

Add bill payment deletion
Add helpers for other line details to line model.