add AppVeyor and SourceLink support

The pull request isn't complete yet, but it is a start and all I have time for tonight.
https://ci-beta.appveyor.com/project/ctaggart/fake

The FSharp.Data pull request https://github.com/fsharp/FSharp.Data/pull/479 is more complete if you wish to look at it for comparison. I have not added the #if Mono stuff yet. I'm not sure how you want to do versions. I've been doing CI builds with the current date in the prerelease. It's up to you. Just thought I'd get feedback.