Add 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 adds the methods required to the PlayerDeathEvent for plugins
to be able to incorporate the behaviour mentioned as a simple boolean
flag.