Added remove command, view now works offline.

- View has been changed to require the world and the player. This allows
  offline mode viewing.
- A restriction on view has been placed. View requires the target to have
  logged in at least once for viewing (this is due to the boilerplate
  nature of Bukkit's inventory structure). There is no way to get around
  this.
- Remove works like view and can delete offline backpacks. It doesn't
  require an initial login.
Signed-off-by: Chris Sanders <zidane@outlook.com>