Added stackable items: coins
Added z-levels.

Currently we have two levels: surface and shallow caves.
Each level has a tile generator, an npc generator, and an item generator.
 - surface does trees and water but no npcs or items
 - shallow caves have only rats and basic (brown) items

Also added stairs which will generate their partner on use.
There's currently no checking if where you end up will actually be useful...
Added seeds to level, now all levels are generated via seed.