Tidy up commands.

- The execute() method has had its docs redefined to reflect how the
  CommandHandler now handles the return value of the method. That is,
  if the method returns false, the CommandHandler prints the usage
  message and description of the command to the command sender.
- Commands now only return false when they should, according to the
  updated docs of the execute() method.
- Most commands have been tidied up a bit, i.e. they've had some
  unnecessary code removed in the advent of the new usage printing
  of the CommandHandler.
- Some commands have had their patterns altered slightly.
- The order of command registration now matters. That is, the order
  commands are registered in will be the order in which the help
  screen will display the command usages and descriptions.
- The help screen now shows ordered commands, and separates the
  types into user, admin and setup sections for a better overview.