coal alloy furnace to return the buckets

![image](https://cloud.githubusercontent.com/assets/2834/3628747/73618ab0-0e97-11e4-8b1c-fb3b86a111b3.png)


The fuel-fired alloy furnace didn't return my bucket after I used a lava bucket as fuel!

Here's my attempt at fixing it - it basically mimics the way it's done in the normal furnace here: https://github.com/minetest/minetest_game/blob/master/mods/default/nodes.lua#L1194

Made a quick test and seems to work (now returns the bucket if used). It also seems to work when using a coal lump (when I used a stack of 9, I got a stack of 8 back).