Added Appveyor configuration.

Fixes #236. Only master branch is built for now to avoid overwriting Travis' status in pull requests. The appropriate badge would have to be added by a repo owner to `README.md` because I don't know the project ID on Appveyor.
It'd be (slightly) faster, but we'd miss on potentially more test failures. I thought having a complete list of failing tests would be nicer than the automated build being 20 seconds faster.