add getAllCompletedTasks to allow clear completed
Merge branch 'master' of https://github.com/cs2103aug2014-w13-4j/main into storage-improvement-chuyu
check add/update time interval
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add check for conditional tasks
Change tabs to spaces
Add interval to tree when undoing deleted task, and remove interval from tree when deleting task
fix the typo
resolve merge conflicts
Fix bug in generating new task storage instance
Merge remote-tracking branch 'origin/storage-improvement-chuyu'