Add continuous integration testing on Windows

As noted in https://github.com/JanusGraph/janusgraph/issues/1019, our tests currently fail on Windows, so we should add testing specifically for the Windows platform to ensure that they pass and avoid any regressions.

[AppVeyor][appveyor] is one such option for a hosted Windows CI tool which is [free for open-source projects][appveyor-pricing].

[appveyor]: https://www.appveyor.com/
[appveyor-pricing]: https://www.appveyor.com/pricing/

Start by reading issue #1019 to understand the Windows test failures and the expected coverage. Review AppVeyor's open-source CI documentation and the repository's current test setup, then configure Windows testing so the project tests run successfully and future regressions are detected.

Written by the indexing model from the issue text.