Delete unused import
implement getSubtask() and addSubtask(Task task) method
update the search method
Pass the unit test for createFile();
Pass the unit test for setFile(String);
Pass the unit test for addTask(Task) and deleteTask(Task);
Pass the unit test for updateTask(oriTask,newTask)
add clear before every unit test
update convetJSONToTask method
Pass the unit test for search(int [])
Major update with important additions
- added access to original user input from command objects
- used format from UI
- added accessible commands type from Command class
- moved hasMissingArgs from Command subclasses to superclass
- implemented redo, help and exit commands
removed binary for merge
Merge remote-tracking branch 'origin/Storage'

Conflicts:
	CS2103T_ZombieTask_DatabaseAPI/.classpath
	CS2103T_ZombieTask_DatabaseAPI/bin/ZombieTask_StorageAPI/StorageAPI.class
	CS2103T_ZombieTask_DatabaseAPI/bin/ZombieTask_StorageAPI/Task.class
	CS2103T_ZombieTask_DatabaseAPI/src/ZombieTask_StorageAPI/StorageAPI.java
	CS2103T_ZombieTask_DatabaseAPI/src/ZombieTask_StorageAPI/Task.java