Add support for AppVeyor CI

[speps]

You wouldn't have been able to do this with Travis CI as they only run OSX and Ubuntu. With AppVeyor it seems to work perfectly, on my fork at least : https://ci.appveyor.com/project/speps/niftyplugins

I can explain directly how to setup the environments, etc. so you can create new releases from the artifacts directly, it's very straightforward :)

[speps]

I had to delete the fork I did, but it should work.