20110820

Continued to work on docs/tickets/47.

• watch_calendars now deletes events in db if they have been deleted from remote calendar.

• New model lino_xl.lib.cal.AttendanceRole. New field Attendance.role.

• New choicelist lino_xl.lib.cal.utils.AttendanceStatus. New field Attendance.status.

• Removed field Attendance.confirmed.

Recurrent events are not yet handled at all. Check-in 20110820