Updating blocks and items

Added new blocks/items and added constants to existing blocks/items for
clearer recipe making.

src/pocketmine/block/DoubleSlab.php

@@ -49,7 +49,7 @@ class DoubleSlab extends Solid{
 			4 => "Brick",
 			5 => "Stone Brick",
 			6 => "Quartz",
-			7 => "",
+			7 => "Nether Brick",
 		];
 		return "Double " . $names[$this->meta & 0x07] . " Slab";
 	}