Added support for Action delegates in TestCase class

[moodmosaic]

This could probably address #16.

Please notice that, in the files changed, I didn't have to return a different ITestCommand instance as suggested in the description.

The following tests passed by only adapting Action to Action<object> in the TestCase class:

[FirstClassTests]
public static IEnumerable<ITestCase> YieldFirstClassTests()
{
    yield return new TestCase(() => Assert.Equal(1, 1));
    yield return new TestCase(() => Assert.Equal(2, 2));
    yield return new TestCase(() => Assert.Equal(3, 3));
}

[ploeh]

Thank you for your contribution! It's now live as Exude 0.2.0.