Added optional Culture parameter to NumberToWords

For me it's something different as with DateTime.Humanize. In it we use the resources defined in our assembly and in here we utilize the standard functionality of the framework.
For DateTime.Humanize our implementation defined that the localization for region specific locale should be the same as for region unspecific one.
For our case we did not made such a decision, so we should use the specific locale.
Further it would preserve the current functionality, as currently `number.ToString()` is used, the specific culture if set as `CurrentUICulture` will be used.