ForAdditionalValues now accepts IEnumerable<string>

This resolves #38.

An example unit test to demonstrate functionality:

	var stringValuesToTry = new List<string> { "1", "3" };
	var valuesToTry = new List<int> { 1, 3 };

	var sut = new EqualitySieve<ABusinessObject>().ForProperty(x => x.AnInt)
		.ForAdditionalValues(stringValuesToTry);

	sut.AcceptableValues.Should().BeEquivalentTo(valuesToTry);

	var compiled = sut.ToCompiledExpression();
	compiled.Invoke(ABusinessObjectWithAnIntOf1).Should().BeTrue();
	compiled.Invoke(ABusinessObjectWithAnIntOf2).Should().BeFalse();
	compiled.Invoke(ABusinessObjectWithAnIntOf3).Should().BeTrue();