Fix mixup of node names/drawtypes for signs

The node name for the sign was "sign_wall", but it was being drawn as a
yard sign.  This fixes it by adding a real yard sign, changing the node
names, and adding an alias.
fix texture surface glitches in water and lava
give furnace separate side/top/bottom textures (copies of stone.png)
Merge pull request #2 from VanessaE/master

fix bugs in signs nodes, water/lava surfaces