Allow quantites to be sepcified for requires_(equipped)_item in powers

Formatting looks like this:
```
requires_item=10,25

requires_equipped_item=112,1
```

- requires_item will consume 1 item by default
- requires_equipped_item won't consume any item by default, but will
consume a maximum of 1 item if specified

Items no longer need to be of the "consumable" type to trigger a power
that consumes them. This let's us have a power-tree power that can
consume gold.
Allow buff powers to drop loot

Syntax in power definitions is the same as events and enemy definitions.
Make it possible to spawn loot from hazards
Rename checkConsumable() to checkUsedItems() in GameStatePlay
Merge pull request #1190 from dorkster/power_loot

Allow powers to drop loot