HasNext

Hi Harri,

have added `hasNext` method to check if next available date is present. Personally, I find throwing an error in `next` method is not a good idea, while exceptions are not welcome in Javascript. Better would be just to return `null` in that case. But it is my imho anyway.  I just added `hasNext` method, so that there is no need to wrap my code into `try..catch`

Cheers, Alex