Accomodate for early workers

Hi,

thanks for this package, this has saved me today. Working early in the morning, I found one hole, I guess.

The code checks for the case that the time delta starts after business hours.
Extending the check to also check of the delta actually stops before before
business hours.

If that doesn't happen, a timespan in the early hours will have the whole day get counted.

Best!
Alex
Python tests say it's invalid, might be a 2.6 thing. All fail though.