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

Sorry for the big commit. List of actions:
1. Refactored AddCommand to use simpler constructor. (Will do this for the other commands as well).
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.