Implement BlockPistonExtendEvent (#1013)

* Implement BlockPistonExtendEvent firing (addresses #922)

* Fix code style

src/main/java/net/glowstone/block/blocktype/BlockPiston.java

@@ -2,7 +2,9 @@
 
 import java.util.ArrayList;
 import java.util.List;
+
 import lombok.Getter;
+import net.glowstone.EventFactory;
 import net.glowstone.GlowWorld;
 import net.glowstone.block.GlowBlock;
 import net.glowstone.chunk.GlowChunk;
@@ -14,6 +16,7 @@
 import org.bukkit.World;
 import org.bukkit.block.Block;
 import org.bukkit.block.BlockFace;
+import org.bukkit.event.block.BlockPistonExtendEvent;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.material.PistonBaseMaterial;
 
@@ -96,6 +99,14 @@ public void onRedstoneUpdate(GlowBlock me) {
                 blocks.add(block);
             }
 
+            BlockPistonExtendEvent event = EventFactory.getInstance().callEvent(
+                    new BlockPistonExtendEvent(me, blocks, pistonBlockFace)
+            );
+
+            if (event.isCancelled()) {
+                return;
+            }
+
             world.getRawPlayers().stream().filter(player -> player.canSeeChunk(chunkKey))
                 .forEach(player -> player.getSession().send(message));
             world.playSound(me.getLocation(), Sound.BLOCK_PISTON_EXTEND, SoundCategory.BLOCKS, 0.5f,