General improvements on commands
Rename isTargetOp in CommandBase to isTargetNonOp and change the way it works
Disallow /defaultgamemode, /difficulty, /gamerule, /setidletimeout, /spreadplayers, and /setworldspawn for non-owners (Fixes #49)
Block /effect, /enchant, and /achievement targeting non-OPs (Fixes #51)
Improve permissions so commands that OPs can't use at all don't show up in /help (Fixes #46)
Refactor ItemStack by renaming some stuff
Add /item (/i)
Allow /list for all players (Fixes #52)
Make canCommandSenderUseCommand not terrible by refactoring it to use an list instead of a huge if statement (Fixes #37)