Set up CI and build script for Linux/MacOS

[christianspecht]

...via Travis?

Example: Chocolatey

[christianspecht]

AppVeyor support this now as well:
https://www.appveyor.com/blog/2018/03/06/appveyor-for-linux/

[christianspecht]

Some parts of the documentation need additional info as well, e.g. the backup directory

[christianspecht]

Environment variables need to be saved as secrets in the repo settings.

Problem: "Secret names must not start with the GITHUB_ prefix."
So we'd need to rename the environment variables here (in the code and on AppVeyor)