Added optional Culture parameter to NumberToWords

@mexx I see your point. To work this around, we may pass the "culture" to the HebrewNumberToWordsConverter's constructor. But this means, that object have to be created each time number.ToWords() is called. Another option might be to pass "culture" to ConvertToOrdinal calls. Again, we discussed pros/cons in DateTime.Humanize()'s thread.

Personally, i do not think there is any real problem with creating new object on each call.
So, if @MehdiK agrees on this, i will change it accordingly.