[u_inv] Add unified_inventory as a subtree

mods/unified_inventory/README.md

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+Unified inventory
+=================
+
+Unified Inventory replaces the default survival and creative inventory.
+It adds a nicer interface and a number of features, such as a crafting guide.
+
+License
+=======
+
+Copyright (C) 2012-2014 Maciej Kasatkin (RealBadAngel)
+
+Unified inventory code is licensed under the GNU LGPLv2+.
+
+Licenses for textures:
+
+VanessaE: (WTFPL)
+  * ui\_group.png
+
+RealBadAngel: (WTFPL)
+  * Everything else.
+
+