Added a Brewing API

The commits say Potion API. Please ignore those.

Usage of this API:

1. To add a new potion ingredient, have your item class implement IPotionIngredient. You will implement a method that returns whether the ingredient can be brewed into the passed ItemStack, and one that returns an ItemStack as the result of brewing the ingredient into the passed ItemStack.
2. To make it possible for an item to be placed into the bottom slots of a brewing stand, just use myItem.setBrewable(true).