Add TYPE_FLEXITIME and TYPE_SPECIAL
Fix use of lambdas for bound context menus
Fix minor coding style problems detected by flake8
Add school holidays for 2015
Remove trailing spaces
Sum holidays for the current user
Do not try to sum for non-existing users
Add attributes startHalfDay and endHalfDay
Do not sum across year boundaries
Add columns start_half_day and end_half_day
Create checkboxes for selecting half a day
Prevent zero length holidays