Add client, models and tests

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.