Changes to Commands

Made some minor changes to the way that we register commands with
Minecraft. Also changed the way that we generate onCommandUsage()
strings so that the style of our outputs is consistent with that of
Minecraft and Forge commands. It seems the way the messages were
generated changed for MC 1.6. Finally, removed compareTo() from
DDCommandBase since it was completely unnecessary (CommandBase already
does the same) and casting objects to CommandBase was causing errors
with some Chicken Bones commands that directly implemented ICommand.