Add CustomModRecipe implementing Bukkit API for non-vanilla recipe types

Now returned from recipe iterator instead of null, for modded recipes.
CustomModRecipe implements org.bukkit.inventory.Recipe, so plugins can
call getResult() from the Bukkit API to get MCP getRecipeOutput().

Advanced plugins can call getHandle() to get the internal IRecipe object
and downcast to specific modded recipe types to obtain further information.

Improves #158.