Added Unit Testing Project - MVCUnitTesting.Tests

The lesson wanted it to be named Unit Tests, but I think the convention is to
use the target project name and add ".Tests" to that name for the new project.
First Unit Test - CustomerUnitTests.cs

This is the first testing class. Turns out it's already bad. Something about
the ActionResult type being returned being something that won't work without
some effort. Rather than do that, he wants the return type changed. This
checkin also downloads the NuGet package for NUnit.