Add ArmorStand Item Meta

This is adds basic item meta for armor stands. It does not add all
possible metadata however.

There are armor, hand, and equipment types, as well as position data
that can also be added here. This initial addition should serve a
starting point for future additions in this area.

Fixes GH-559

paper-server/src/test/java/org/bukkit/craftbukkit/inventory/ItemMetaTest.java

@@ -365,6 +365,7 @@ public class ItemMetaTest {
                     final CraftMetaArmorStand meta = (CraftMetaArmorStand) cleanStack.getItemMeta();
                     meta.entityTag = new CompoundTag();
                     meta.entityTag.putBoolean("Small", true);
+                    meta.setInvisible(true); // Paper
                     cleanStack.setItemMeta(meta);
                     return cleanStack;
                 }