Enable resolving projects to resolve references from packages.config.

This change introduces the new build tools package version of 1.0.17.

Currently the ResolveNuGetPackage task doesn't properly set Private on the
resolved references so not all libraries like xunit are getting placed in
the output folder. That causes runtime issues so to avoid that this nuget
resolution is turned off in the XunitTraitsDiscoverers.csproj which will
cause the xunit libraries to end up in our tests folder and can be used by
all the tests.
Convert XUnitTraitsDiscovers project to package resolution
Convert System.Console to package resolution
Switch all projects to resolve references from packages.config.
Merge pull request #310 from weshaggard/NugetRefs

Switch all projects to resolve references from package.config