Several bugfixes and additional features

- Additional cast (LivingEntity) is now required (Bukkit 1.7.2+).
- Added sounds when killing an enemy with the bow or sword.
- Only add one arrow after getting a kill and update the inventory.
- Send winner message to the global lobby only to prevent confusion/spam in other arenas or worlds.
- Added Economy support and settings entry "award-amount" to specifiy the amount you can win.
- Don't forget to add back the arrow if it's missing after killing an enemy without the bow