Add continuous integration for windows

We're merging in changes which add support for windows. When that support breaks we won't notice for some time unless we add continuous integration. Unfortunately, Travis doesn't support Windows.

Some options:
- [Visual Studio Team Services Continuous Integration](https://www.visualstudio.com/team-services/continuous-integration/)
- [AppVeyor](https://github.com/marketplace/appveyor)

AppVeyor appears to have tighter github integration (it'll set the build status so we can enforce that PRs don't break Windows).