Add AppVeyor for Windows testing.

TODO:

- [ ] Change the badge code before this is merged

/CC @addyosmani: don't merge this yet; we need you to add grunt-uncss in AppVeyor yourself and post the badge code (or add me as a collaborator).

Example of how things will be: https://ci.appveyor.com/project/XhmikosR/grunt-uncss