Added toggleStatus method to toggle the completion status of a task.
runCommand will now call the appropriate methods for add/edit/delete/mark by ID and undo after getting the parsed user command.
Added code to complete saveToDrive in DonLogic.
Extracted "Start date" and "End date" strings into constants.
Changed @author tags in source files in the logic package to follow the V0.4 requirements.
Added empty lines to make reading runCommand easier.
Added placeholder for findTaskRange to search for tasks within 2 given dates.
Added placeholder for getHelp.