Forgot to alter VL.
Correct type of placed block, autosign uses Material.SIGN now.
Avoid sign duplication due to a bug in CraftBukkit.

The cancelled BlockPlaceEvent will lead to an extra sign being dropped,
while the item in hand stays. Odd enough, the cactus is removed before
the BlockPlaceEvent - need to check if a ticket exists for CraftBukkit
already...