Added optional Culture parameter to NumberToWords

@hazzik I do not think that cache is that good idea here, as we then have to take care of race conditions and stuff like that, which will make the library less usable with multithreaded server code (this is my primary area of interest and that's why i initially wanted to have this explicit culture support). 

To be fair, i do not think that introducing locks just to avoid passing additional parameter makes any viable sense.