Add AppVeyor CI

[zvirja]

We definitely need CI for this project, otherwise it's very dangerous to merge the PRs. Everything is ready for CI (configuration file is already present), you need to do a couple of things only:

• Go to AppVeyor and create a build for this repo (a few clicks).
• If you wish to enable automatic release after you create a tag (via GitHub UI), please update the appveyor.yml file and specify the NuGet API Key (encrypted) to the NUGET_API_KEY variable.

[moodmosaic]

I always run a build on my local before merging a PR, plus this repo didn't receive that many PRs until now, but yeah, this can be useful 👍

[moodmosaic]

I think we need something like this just to be able to grab the generated NuGet Packages and publish them. Personally, I no longer use Visual Studio and having troubles running the build scripts as-is from #30.

Even on Windows, with .NET Core installed, build scripts don't seem to work as per #42.

[zvirja]

Answered in #42 - build scripts should be rewritten if VS installation is missing, so we use dotnet instead of MsBuild