Allow Groupdate users to specify an hour of the day that is the conventional start of the day

So the use case is: our product deals with hailing taxis.  We want to see how many hails we get in "one day," for example.  But if we regard the start of the day as midnight, on, say, New Year's Eve, we see two days with big traffic when really the deal is that there was a single event-based demand surge.

As a result, we conventionally regard days as starting and ending at 4am, which creates results when grouped by day that more correctly isolate single-day demand surges.  I've enhanced Groupdate to take a day_start option to most of its methods, allowing you to specify an hour (4, in our case) when your days conventionally start.

The change should be innocuous to anyone who doesn't want to use the option.