Add models and get_batch_errors method

See merge request !9

- Both get_batch_errors and get_batch_complete now return objects
  unless the client parameter return_objects is set to False
- Tests for models, endpoints and client methods are included.

OM46, del 1: Support batch_complete endpoint from setra

See merge request !7

Make Batch.period optional

See merge request !8

Validation rule 12 on Batches in SETRA sets the default value before
transfer to Unit4ERP(UBW) if it is not set. The attribute is thus not
required.

Make vouchertype optional

See merge request !6

There are validation rules in SETRA that sets this value to a default
value set in the parameters table of SETRA. For this to be possible
we must allow users to leave out the value.

Fixed dataformat. Moved transactions list onto voucher.

See merge request !5

Also updated and fixed tests, fixtures and readme

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