And added two new tests

Added lots of tests. And fixed some issues with the client, discovered while testing

See merge request !4

Make return_response a param with default value. Related to BOOK-17

See merge request !3

This gives us more options for how to use the function, and makes setra client easier to use in Bookitlab-api (BOOK-17)

Remove unused fields

See merge request !2

batchid- and voucherno_interface are not used.

They where originally added because someone thought we should enforce
relations in software 

This allows us to catch more errors that can occour when transferring
data to Setra.

Make some types optional, not delivered my ompostering

See merge request !1

Introduces SetraClient. The client is made
for use against the SETRA(SEntralt TRAnsaksjonslager) API.

There are models for Batch, Voucher, Transaction and Multi, where Multi is
a combination of the preceding, The Multi model validates that the batchid
of the voucher matches the batch included, and that the voucherid of the
transactions matches one of the included vouchers.

Tests have also been included for the client itself, the endpoints, and
the models.

Configuration for tox, Jenkins and gitlab-ci are included for running
tests and building the client.