Use Bukkit Material constant- AIR should be the same as AIR, I hope!
Add some fixes and safety checks mainly to do with early startup/initialization.
Put a slight-hack in to make sure certain attributes get processed before custom ones.
Refactor to change lore assignment to happen before attributes update data.

The only impact this has API-wise is that ItemAttr.onStatusLoreRequest
no longer takes the ItemStack target, since it is not created yet. Most
methods that call StockItem.getItem now build the lore in advance if
they have custom lore.