Visualizations play better with signs.

Previously could cause clients to see blank signs until they refreshed
the sign block (on relog or right click).  Now signs won't be replaced
with visualization blocks.

src/me/ryanhamshire/GriefPrevention/Visualization.java

@@ -282,6 +282,9 @@ public class Visualization
 		return (	block.getType() != Material.SNOW && (
 		            block.getType() == Material.AIR ||
 					block.getType() == Material.FENCE ||
+					block.getType() == Material.SIGN ||
+					block.getType() == Material.SIGN_POST ||
+					block.getType() == Material.WALL_SIGN ||
 					(waterIsTransparent && block.getType() == Material.STATIONARY_WATER) || 
 					block.getType().isTransparent()));
 	}