Add a new method to Time

Hey guys,

I added the method `first_business_day` in a application that I work and I think that it could be pretty useful for other people.
This method returns the time of the next business day or itself if it is a business date already.
The difference between `first_business_day` and `roll_forward` is that the new method cares about only with the day and not with the time.

I don't know if I picked the best method name.

Well, let me know if I can improve anything. Best regards.