Add MSBuild 15.1 discovery capabilities to NuGet.exe

[drewgillies]

Fixes NuGet/Home#3408. @alpaix @dtivel @emgarten
Notables:

• This starts to address the broader stories of how to restore in a post-GAC MSBuild world and how to restore when VS14 is not present.
• We no longer take dependencies on Microsoft.Build or Microsoft.Build.Framework DLLs in nuget.exe - these are all discovered at runtime and loaded dynamically
• SxS (VS15+) and classic toolsets are abstracted into a new type MsBuildToolset, which is used for all toolset selection/ordering/path discovery
• Projectfactory now has load failsafes as these dynamic loads were failing to look in directory of last load for dependent assembly Microsoft.Build.Framework.dll and resources
• Incorporates recent changes to support Mono
• New tests written to cover SxS and mixed legacy/SxS discovery scenarios
• MSBuild 12 appears to no longer install with VS14, so I've repurposed the MSBuild 12 test to 15.1, and added a skip to it until CI machines run MSBuild 15.1.

[drewgillies]

There's a better way to bootstrap this. Pulling the PR for now.