This fixes compiling on OS X and makes make_release.py create a standalone app bundle on OS X

This fixes #1207. The important parts for the standalone app bundle stuff are the following:
- libfreetype (and libpng if needed) are copied into the app bundle.
- The teeworlds binary and the libraries are modified with install_name_tool, so the libraries in the bundle can be found by them.
- For this we also need to pass ```-rpath,@loader_path/../Frameworks``` to the linker, so that the framework folder in the app bundle is actually in the dynamic library search path of the teeworlds binary. We do this by adding the flag ```-Wl,-rpath,@loader_path/../Frameworks```.

This also makes make_release.py work on other systems again (it did not, because readme.txt got renamed to readme.md).