Redstone controlled factories

This update provides redstone control for factories by connecting the redstone to the furnace block.

To make a factory redstone-sensitive it *must* have a lever attached to the furnace. Without this, it will not be controlled by the redstone. A redstone pulse to redstone wire next to the furnace (any side) will turn the factory on if it's ready. Likewise, flipping the lever or pressing an additional button attached to the furnace block will count as a redstone pulse. *At the moment it will not respond to a repeater unless you also stick redstone on it*.

When the factory starts for any reason, the lever is switched to the on position. This sends out a redstone signal which can identify that the factory is running. When turned off for any reason, the factory is turned off. At the moment, turning the lever off by hand does nothing (hope to make this cancel the job in another update).

This can be combined with hoppers, etc, to build sophisticated factory control systems.