Added spawn_monster file. "Bands" of creatures can spawn.
A band has a specific leader that they follow around while they can't see anything to fight.
randomize spell timer to make spellcasters more varied.
fixed a bug where droppped items didn't stack correctly.
Item spawning on all branch levels.
Rarity attribute for items, the chance they will spawn are based on this rarity. spawn_item module will also specifiy rarities for each item type. Currently the pick algorithm is really naive; I'll have to fix that sometimes later.