Added optional Culture parameter to NumberToWords

You are right. Let me explain my thoughts here:

1. We've discussed similar issue while i was working on DateTime.Humanize. It seems that @MehdiK decided that it's not important and merged DateTime.Humanize as it was.

2. The only reason we use "_culture" here is that HebrewNumberToWordsConverter misses ConvertToOrdinal implementation. Once that is implemented, issue will disappear.