Switch code license to zlib, clarify art license (a bit)

Compromise for issue Siile#13. Because some of the art is commercial, for now we may as well just relicense the _code,_ and figure out the art later on.

Everyone who has contributed code has already agreed to relicense their code from CC BY-NC-SA to `zlib` (see Siile#13).

license.txt

@@ -1,29 +1,36 @@
-Teeworlds Copyright (C) 2007-2014 Magnus Auvinen
-Ninslash Copyright (C) 2016 Juho Syrj�nen
+Teeworlds Copyright (c) 2007-2014 Magnus Auvinen
+Ninslash Copyright (c) 2016 Juho Syrjänen
 
 This software is provided 'as-is', without any express or implied
-warranty.  In no event will the authors be held liable for any damages
+warranty. In no event will the authors be held liable for any damages
 arising from the use of this software.
 
-You are free to:
-
-Share � copy and redistribute the material in any medium or format
-Adapt � remix, transform, and build upon the material
-
-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.
-NonCommercial � You may not use the material for commercial purposes.
-ShareAlike � If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
 
+1. The origin of this software must not be misrepresented; you must not
+   claim that you wrote the original software. If you use this software
+   in a product, an acknowledgment in the product documentation would be
+   appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be
+   misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
 
 ------------------------------------------------------------------------
 
-All content under 'data' except the font (which has its own license) and all content under 'src' is
-released under CC BY-NC-SA 4.0 (https://creativecommons.org/licenses/by-nc-sa/4.0/).
+Licenses for content under 'data' varies; some is freely licensed under
+the CC-BY-SA 3.0 license [1], some is nonfree. Until license documentation
+for this project improves, it is recommended to look for the same file in
+the original Teeworlds source code [2], which will have better
+documentation. Any original art made for Ninslash is licensed freely under
+the CC-BY-SA 3.0 license [1].
+
+[1]: https://creativecommons.org/licenses/by-sa/3.0/
+[2]: https://github.com/teeworlds/teeworlds
 
 ------------------------------------------------------------------------
 
 IMPORTANT NOTE! The source under src/engine/external are stripped
 libraries with their own licenses. Mostly BSD or zlib/libpng license but
-check the individual libraries.
+check the individual libraries.