add `appveyor.yml` file for Windows CI testing

We should consider using [appveyor](http://appveyor.com) for Windows CI testing. This is basically the same `appveyor.yml` file that I've been using in some of my recent native build projects (see https://ci.appveyor.com/project/TooTallNate/node-weak).

If we want this active then we'll have to merge this PR and then @rvagg will need to register an account on the website (it's free) and activate this project via the `New Project` button.

Thoughts?