Enable Appveyor for unit testing

[kilasuit]

To enable the repository to be able to use Appveyor to run the included Pester tests this would have to be added by a member of the Docker Appveyor team which was something that I could not do.

The Appyeyor.yml file is all set up and ready can could be used as well easily enough to publish newly merged PR's given the right tweeking

[pdevine]

@kilasuit can you describe the use case in a little more depth? I don't really know what the benefits of Appveyor are and am trying to understand it better.