Improved usability by only requiring one namepsace import

Now the programmer only needs to add the TestingTools namespace:

```csharp
using TestingTools;
using NUnit.Framework;

[TestFixture]
public class MyTest {
  [TestMethod]
  public void MyTest() {
    // Arrange
    string expected = "new";
    // Act
    string actual = "n" + "ew";
    // Assert
    Verify.That(actual).IsEqualTo(expected);
  }
}
```