Refactored AddCommand to use simpler constructor. Added CommandExecution...

Sorry for the big commit. List of actions:
1. Refactored AddCommand, EditCommand, DeleteCommand, DisplayCommand to use simpler constructor. 
2. Added CommandExecutionException for ICommand.execute. The exception message will be displayed to the user.
3. Separated JUnit Tests into a separate physical folder.
4. Added ListTestSuite class to run all JUnit tests in one click.
5. Refactored startTime and endTime to startDate and endDate, respectively.