[FIX] project_issue, resource: fixed computation of hours / days to open...

[FIX] project_issue, resource: fixed computation of hours / days to open (assign) or close an issue. Those fields now have a conditional store, because they were not recomputed even when changing date_open and date_closed. Changing the stage now correctly changes the date_closed. Also added some fixes in resource module, the date was not correctly taken into account when computing the number of working hours between two dates.