NuGet 2.8 Automatic Package Restore

Switch to using the recommended 'Automatic Package Restore' method, as provided by NuGet 2.7+ - http://docs.nuget.org/docs/reference/package-restore.

For command-line builds, the build.proj calls nuget.exe first to restore packages before the main solution is built (based on http://docs.nuget.org/docs/reference/package-restore-with-team-build)

Fixes VS2013 compatibility bug #18 at the same time.