Add AppVeyor testing for Windows

Things we need before merging

- [ ] Someone should add JSHint in AppVeyor themselves and should post the badge code so that I update the patch.
- [ ] Enable cache for the npm modules; this requires the build to pass first to actually have an effect, see #1931 too.

Example: https://ci.appveyor.com/project/XhmikosR/jshint

/CC @caitp @jugglinmike @rwaldron