Support for MSBuild Core

Target CoreCLR with the MSBuild task and reference MSBuild Core assemblies.

Remaining work items:

• libgit2sharp needs to be a bit smarter about adding the native binaries directory to the PATH than just to use AppContext.BaseDirectory because that doesn’t work for an MSBuild Task that uses it.
• figure out how to automate the build of a NuPkg with all the right files in place.
• Load the right MSBuild Tasks assembly based on whether MSBuild is running on .NET Fx or .NET Core
• Re-implement ThisAssembly code generation with Roslyn APIs
• Get tests running again (on both net45 and .NET Core)
• Get this building on appveyor
• Check that NPM package contains a nbgv.nuget folder that contains a subset of what is in the nupkg rather than a bunch of facade assemblies under build\msbuildcore

To work on Linux:

• Fix native libgit2 library loading in libgit2sharp (https://github.com/AArnott/Nerdbank.GitVersioning/issues/112)

[AArnott]

OK, I have a semi-supportable package for folks to use:

nuget feed: https://www.myget.org/F/aarnott/api/v2/package package version: 1.6.1-beta-g4dc3644c72

Of course later versions in the 1.6.x family are probably better when they become available.

[AArnott]

Alright: I’ve hacked together a NuGet package that works on dotnet build. I tested on Windows, but I believe it will work on other operating systems as well.