Nullable types support

[davideicardi]

It should be possible to write:

        var target = new Interpreter();

        var lambda = target.Parse("null", typeof(int?));
        Assert.AreEqual(typeof(int?), lambda.ReturnType);
        Assert.IsNull(lambda.Invoke());

        lambda = target.Parse("4651", typeof(int?));
        Assert.AreEqual(typeof(int?), lambda.ReturnType);
        Assert.AreEqual(4651, lambda.Invoke());