Add charcoal (burned tree)

Adds charcoal blocks for :tree and jungletree, crafted by smelting. A charcoal block is not as good a fuel as coal, but is better than tree.

![screenshot_707739139](https://f.cloud.github.com/assets/3813235/1154694/cceea84a-1f52-11e3-9b22-c7f91d33830e.png)