Improve /bec viewinv

- Added betterenderchest.command.viewinv.self permission to only view
your own Ender Chest.
- /enderchest will now fall back on /bec viewinv if /bec openinv isn't
available.
- The message that you can only open your own Ender Chest can now be
translated.
Fix drop code

Now the Ender Chest properly drops itself.
Fix /bec reload

- Removed unnecessary BetterEnderChestPlugin reference. No need to code
against the implementation.