AppVeyor CI

[sushihangover]

Add AppVeyor CI to project for Window build testing

[sushihangover]

Some references for AppVeyor setup:

http://www.codeproject.com/Articles/815565/How-to-build-Mono-on-Windows

https://github.com/DynamicDevices/mono/blob/master/appveyor.yml

https://ci.appveyor.com/project/ajlennon/mono-817

http://www.appveyor.com/docs/lang/xamarin