Move nyancats into a separate mod

Nyancats are independent in the default mod. Nothing else uses them or
their code. Separating it into a separate mod makes it easier for
subgames to remove them. It also makes it easier for a mod to depend
on nyancats, as lots of subgames don't have them.

Default/mapgen.lua: Register biomes, ores and decorations in
singlenode mapgen. These were never disabled anyway because
singlenode was removed from the world creation menu.

mods/nyancat/README.txt

@@ -0,0 +1,29 @@
+Minetest Game mod: nyancat
+==========================
+
+License of source code:
+-----------------------
+Copyright (C) 2011-2012 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+http://www.gnu.org/licenses/lgpl-2.1.html
+
+License of media (textures and sounds)
+--------------------------------------
+Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
+http://creativecommons.org/licenses/by-sa/3.0/
+
+Authors of media files
+-----------------------
+Everything not listed in here:
+Copyright (C) 2010-2012 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+VanessaE (WTFPL):
+  default_nc_back.png
+  default_nc_front.png
+  default_nc_rb.png
+  default_nc_side.png