Include contributions from recent fork

[klamann]

Hi @philipflohr, I noticed your fork at philipflohr/personio-py, great stuff!

Would you like to open a PR so we can include your changes in the main repo and publish them on PyPI?

[philipflohr]

Hi @klamann, sure that was my idea. Thanks for that api communication basics - this makes adding new functionality really easy.
I'm not yet finished with my work as I'll need all the functions offered by the personio api - there are still some functions missing. Let's discuss the details in the PR.

[philipflohr]

Hi @klamann.
I'll split the code changes into smaller chunks in separate PRs.
The most basic changes are in the PR #9 , #10 and #11

Once these are merged I can provide tho code changes from #8 in logically separated PRs (implementation and tests of absences and attendances together).

[klamann]

awesome, I'll look into those!