Added optional Culture parameter to NumberToWords

> Maybe instead of creating the converters on each call we can register HebrewNumberToWordsConverter with both "he" and "he-IL" passing the culture to the converter to be cached by the converter locally?

Sure, we can do this. But this does not solve issue for the default converter. Obviously, we can not pre-register an instance for each unsupported culture :).

Basically, if you remember my initial implementation of DateTime.Humanize ... i used to pass the culture object directly to the converter's methods. This solves issue with many-small-objects-have-to-be-created + we always can use actual culture instead of made-up one. But then people did not like it actively and we decided to pass the culture to the converter's constructor. So... what i think here... may be that was not that bad idea at all and we can do it that way? :)