Added ClearDone command.
Changed TimePeriod to contain only StartDate and EndDate.
Added MarkUndone command
Added exception handling for MarkDone if String without integer is passed to it.
fixed the reminder problem. now reminder only pop 1 time for a task
added sound alert for the reminder popup
fiexed reminder time delay when you add an immediate task
eg: if at 0500, you add task by 0505, it will not work at first.
now it is fixed
increase reminder display to 8 seconds. changed some text.
Merge remote-tracking branch 'origin/yongkai'
Merge branch 'master' of https://github.com/cs2103aug2014-t11-1j/main into krystal