Change WTFPL to MIT (part 3)

Relicense WTFPL media to CC BY-SA 3.0.
Various corrections and edits.

README.txt

@@ -4,11 +4,23 @@ See license.txt for license information.
 
 Authors of source code
 ----------------------
-Ironzorg (MIT)
-VanessaE (MIT)
+Originally by Ironzorg (MIT) and VanessaE (MIT)
+Various Minetest developers and contributors (MIT)
 
 Authors of media (textures)
 ---------------------------
-Gambit (MIT):
-flowers_mushroom_*.png
-flowers_waterlily.png
+RHRhino (CC BY-SA 3.0):
+  flowers_dandelion_white.png
+  flowers_dandelion_yellow.png
+  flowers_geranium.png
+  flowers_rose.png
+  flowers_tulip.png
+  flowers_viola.png
+
+Gambit (CC BY-SA 3.0):
+  flowers_mushroom_brown.png
+  flowers_mushroom_red.png
+  flowers_waterlily.png
+
+yyt16384 (CC BY-SA 3.0):
+  flowers_waterlily_bottom.png, derived from Gambit's texture

license.txt

@@ -1,9 +1,9 @@
-License of source code and media (textures)
--------------------------------------------
+License of source code
+----------------------
 
 The MIT License (MIT)
-Copyright (C) 2012-2013 Ironzorg, VanessaE
-Copyright (C) 2015-2016 Gambit
+Copyright (C) 2012-2016 Ironzorg, VanessaE
+Copyright (C) 2012-2016 Various Minetest developers and contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of this
 software and associated documentation files (the "Software"), to deal in the Software
@@ -23,3 +23,40 @@ DEALINGS IN THE SOFTWARE.
 
 For more details:
 https://opensource.org/licenses/MIT
+
+
+Licenses of media (textures)
+----------------------------
+
+Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
+Copyright (C) 2014-2016 RHRhino
+Copyright (C) 2015-2016 Gambit
+Copyright (C) 2016 yyt16384
+
+You are free to:
+Share — copy and redistribute the material in any medium or format.
+Adapt — remix, transform, and build upon the material for any purpose, even commercially.
+The licensor cannot revoke these freedoms as long as you follow the license terms.
+
+Under the following terms:
+
+Attribution — You must give appropriate credit, provide a link to the license, and
+indicate if changes were made. You may do so in any reasonable manner, but not in any way
+that suggests the licensor endorses you or your use.
+
+ShareAlike — If you remix, transform, or build upon the material, you must distribute
+your contributions under the same license as the original.
+
+No additional restrictions — You may not apply legal terms or technological measures that
+legally restrict others from doing anything the license permits.
+
+Notices:
+
+You do not have to comply with the license for elements of the material in the public
+domain or where your use is permitted by an applicable exception or limitation.
+No warranties are given. The license may not give you all of the permissions necessary
+for your intended use. For example, other rights such as publicity, privacy, or moral
+rights may limit how you use the material.
+
+For more details:
+http://creativecommons.org/licenses/by-sa/3.0/