Fix:Ensures that patients won't fall through walls when they die

This commit fixes: [issue 947](http://code.google.com/p/corsix-th/issues/detail?id=947)

It adds code to Die.lua:action_die_start to ensure that patients have enough space to fall over when they die.

It also adds a supporting function to World.lua which checks that a tile has no entity on it and (optionally) isn't in a room: World:isTileEmpty()