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.
Add a bit more debug output.

This isn't needed to resolve the problems, but it's useful in other
cases.
Re-target .NET 4.0 in the Tests project.

The build machine doesn't have .NET 4.5 installed (yet), and the Web
project is targetting .NET 4.0 anyway. This should resolve some warnings
in the build log, but I don't think it will resolve the build failure.
More debug output.

Dump the properties hashtable out to the console before each task runs.
Use -Include rather than -Filter.

This appears to be a difference between PowerShell versions: -Filter
works as intended in PowerShell 3 but behaves differently in PowerShell
2. Using -Include has the desired effect on both versions of PowerShell.
Merge pull request #111 from alastairs/build-run-tests

Fix #104: Make the build run the tests