dotnet cli build pipeline and AppVeyor CI pipeline

[ivanz]

This pull request achieves:

• Build pipeline based on dotnet cli: currently side by side with the sln+csproj
• Sets up build+test+nuget packaging pipeline in AppVeyor
• AppVeyor is also integrated in checking new pull requests

All tests pass 👍 , so that should give me a good foundation to start introducing the .Net Standard targets.

Couple of notes:

• The framework targets in the build pipeline are: .NET 3.5, 4.0, 4.5, 4.6
• I haven't created xproj yet, because Visual Studio 2015 doesn't handle project.json without a .NET Standard/Core framework target, but that will be a non-issue once I start porting.

Working towards #280

I will keep the ruby+sln+csproj+msbuild stuff for now until the following two issues are resolved (should not block merging this in):

• The Machine.Specifications.Clr4 assembly is not included in the .nupkg for some reason: https://github.com/dotnet/cli/issues/1290
• Strongly Signed version (doubt this is too important?)

[ivanz]

@einari In case you want to have a review of this 👍

[einari]

Looks good. Added an issue (#295) for the future. I can probably look into that a bit later. Do the honors of merging.. 👍