Windows support and Travis CI

[gogowitsch]

This pull request adds Windows support (details in the commit message).

To prove that I didn't destroy Linux support on the way, I manually tested things and added Travis CI coverage to my fork of this repo. See https://travis-ci.com/Fonata/dorgflow/builds/102212670 for some test output.

@joachim-n If you decide to merge this, you probably have to allow Travis to access your repo. It will set up a hook automatically to run unit tests on each push and each pull request.