Add testing on AppVeyor

[Alxandr]

Add appveyor testing to assert it works on Windows.

[erikd]

@Alxandr If @kazu-yamamoto is not familiar with Appveyor, maybe you could provide a PR.

[kazu-yamamoto]

I don't use Windows and Appveyor.
PR is very welcome.

[Alxandr]

Setting up AppVeyor to do testing on Windows is fairly easy. Just copy the following file to your repo: https://github.com/commercialhaskell/stack/blob/master/appveyor.yml, then go to AppVeyor and sign up (just like you did with Travis). Considering you're using stack that should be all that's needed.

[kazu-yamamoto]

I setup AppVeyor.