Close #4, add ender backpack + more

- Added ender backpack
- Added ItemBackpack.getBackpackItems
- Removed need for "hasItems" item NBT data
- Renamed InventoryWrapper to InventoryStacks
- Added InventoryWrapper, actually wraps around a base inventory
- Added StackUtils.isEmpty
- Added equals for InventoryWrapper and InventoryStacks
And a few smaller changes related to the above ...