Support for night owls: starting a day at later hour, e.g. 6, to properly report late night work

[sfider]

I like to work late at night, even after midnight. Using Watson in such a setting means reports will count part of my work as being done on the next day, while it was done on the same day from my perspective. I've added 'day_start_hour' option to shift time spans used for reports/aggregates to account for me working late.

[ericguirbal]

This addresses #444. Thanks.