Specify MaterialData for Acacia and Dark Oak stairs. Fixes BUKKIT-5037

When Minecraft 1.7 was released, Acacia and Dark Oak Stairs were added.
While Bukkit added them to Material.java, it did not add the MaterialData
mapping of them to Stairs.class.

Currently getAscendingDirection() and other stair-specific functions can
not be used on these new stairs. This commit fixes that by adding the
mapping needed.

Pulled from PR #977

src/main/java/org/bukkit/Material.java

@@ -227,8 +227,8 @@ public enum Material {
     STAINED_GLASS_PANE(160),
     LEAVES_2(161),
     LOG_2(162),
-    ACACIA_STAIRS(163),
-    DARK_OAK_STAIRS(164),
+    ACACIA_STAIRS(163, Stairs.class),
+    DARK_OAK_STAIRS(164, Stairs.class),
     HAY_BLOCK(170),
     CARPET(171),
     HARD_CLAY(172),