Appveyor integration

[nirs]

We want to port the project to Windows, to make it easy to deploy the project in school environment, which may be using non-free software such as Windows (see #179).

The first step would be to have the tests running on windows. We can use the Appveyor CI integration in github.

Tasks:

1. Add appveyor.yml
   See this example:
   https://github.com/nirs/pytest-threadleak/blob/master/appveyor.yml
2. If the tests fail, mark the failing tests as expected failure:
   See http://doc.pytest.org/en/latest/skipping.html
3. If you can fix trivial issues instead of marking the tests as expected
   failures, please do.