Add AppVeyor support for Windows testing.

TODO:

* Change the badge branch before this is merged
* Change default branch to master

/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