resolve typo of method name marTaskComplete()
typo of method name
change deleteTask() return String and add the journal description.
change deleteExistTask test
change update() return String and fix bugs in journal record of delete()
change update test expected string
add markTaskComplete in to execute
change return type of markTaskComplete()
change test case and name of markTaskComplete
add method addUncompletedTask()
change catch IOException to  throw
add method markTaskUncomplete()
add new test case of markTaskUncomplete
integrate mark command to execute()
Hotfix in Logic.executeCommand to pass response of MARK and DELETE back