Add NUnit.Runners package to the solution.

This is a solution-level package, not a project-level package. As such,
it adds itself to the .nuget folder and the solution file.
Add new environment script to set PATH, etc.

And call the script from the run-build script.
Fix #104: Add a Test task to the build script.

Execute the unit tests via nunit-console.
Add a bit of debugging info.

Log out the number of test assemblies found, and their paths.