Allow assigning a modId to a custom itemstack

Currently you can assign a custom modId to Blocks and Items but not custom ItemStacks.
These changes allow for you to do just that.

Also a small change was done to GameRegistry.registerItem(Item, String)
It now returns the Item to be consistent with the other register blocks/items methods.
Merge branch 'master' of https://github.com/ShetiPhian/FML