Fix special key handlind

* Offset special keys by 256
* Simplified the key handlind, now using the GLUT keycodes + offset
  directly

Closes #49
3rd party licenses

A txt file in the doc folder
Merge branch 'master' into Bass
MP3-free Bass

* Changed binaries to the mp3-free version
* Added Bass license to 3rdparty_licenses.txt