Adding ForAdditionalValues() for...additional values support.

This resolves #31.

With this change, we add an additional method: `ForAdditionalValues()`.

This method, which can be used with `IEnumerable<TPropertyType>` (e.g. IEnumerable<int> if the property type is int) or `IEnumerable<string>`, adds additional values to parse.

So, the following now works:

	// both of the below create equality filters for the numbers 1-6
	var example1 = new EqualityFilter<ABusinessObject>()
		.ForProperty(x=>x.AnInt)
		.ForValues(new List<int>{1,2,3})
		.ForAdditionalValues(new List<int>{4,5,6});

	var example2 = new EqualityFilter<ABusinessObject>()
		.ForProperty(x=>x.AnInt)
		.ForValues("1,2,3")
		.ForAdditionalValues("4,5,6");