Implement ability to keep items on death via plugins. Adds BUKKIT-5724

When a player dies their inventory is normally scattered over the the area
in which they died. Plugins should be able to modify this behaviour by
defining whether or not the player's inventory will be dropped on the ground or waiting for the player when they eventually respawn.

This commit implements the methods included in the Bukkit half for the new
behaviour by acting upon the boolean flag. The boolean flag is tested
prior to clearing the inventory as well as prior to dropping the items on
the ground. If the flag is true (indicating "keep inventory"), the items
are not removed from the player's inventory and are not dropped on the
ground.