Corrected COPYING.txt

* AGPLv3 should have been AGPLv3+.
* Elnode license name was incorrect.
* style.css should have been mentioned alongside example.creole
Added missing copyright notices

Added copyright notices to style.css and example.creole, and corrected
copyright of example.creole, which is based on a file by Nic Ferrier
which was released under the GPLv3+.