Add build support for .NET Core

[dsplaisted]

This PR allows us to target .NET Core (#140) and to track progress towards compiling successfully for .NET Core

• Adds configurations to build for .NET Core
• Ports Microsoft.Build.Framework to build successfully for .NET Core
• Starts porting Microsoft.Build to .NET Core
• Adds compilation constants and MSBuild defines for features not available in .NET Core (we will need to add more as we hit them while porting)
• Adds Port-Progress configuration to build against .NET 4.6 but using the features flags targeting .NET Core, so we can successfully compile and track our porting progress with ApiPort. Added batch file to run ApiPort against this build

[dnfclas]

Hi @dsplaisted, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!
You've already signed the contribution license agreement. Thanks!

The agreement was validated by .NET Foundation and real humans are currently evaluating your PR.

TTYL, DNFBOT;

[AndyGerlicher]

LGTM. We'll need to fix the CI build though. It would be good to at least have something in place.