Adding an AppVeyor configuration file to run Windows CI builds

AppVeyor is a Windows CI solution that provides a [free service](http://www.appveyor.com/pricing) for open source projects; this sets up for using this service. Appveyor doesn't provide the extensive buildmatrix that Travis-CI provides (1.9.3 and 2.0.0 with x86 arch and 2.0.0 with x64 arch), but it does provide access to the Windows platform which Travis doesn't.

It will also do [various notifications](http://www.appveyor.com/docs/notifications) (which I haven't set up now to prevent spamming)
The dashboard looks like this for my fork: https://ci.appveyor.com/project/mprins/compass

To get this going on the Compass repo you'd need to:
  - login at https://ci.appveyor.com/signup/free using your Github account
  - use the "+ new project" on top of the page to select and add the Compass project
  - merge this PR

This replaces #1783


Related to this is the possibility to display the build status of both Travis-CI and AppVeyor on PR's using the service provided by @astrofrog running here: http://github-multi-status.herokuapp.com/