Improved the Command - CommandFactory implementation.

* Moved from 'init' method to constructor.
* Expected format for the constructor described in the Command
interface comments.
* Distinguish between exceptions that occur during construction of the
command, log these and inform the user of an error; and exception
occurring during the execution of the command.
* Better error handling for commands.

To do:
* Add 'help' method to the Command interface.
* Catch IllegalArgumentException and automatically display help()
information to the user.