Added WorldGuardHandler & WorldGuardClass

* WorldGuardHandler does all the work to do with WorldGuard (obviously)
* WorldGuardClass is a bridge between the actual plugin.
* If WorldGuard is not found, WorldGuardHandler will also not be
initialised.
* WorldGuardClass has checkWorldGuard(), checkWGCustomFlags() and
initialiseWGHandler().
* WorldGuardHandler holds all WorldGuard needed methods.
Updated to integrate with WorldGuard

* Request:
http://dev.bukkit.org/server-mods/inventory-drop-chance/?comment=7