Achievements as a Specific Class

I've started implementing achievements as a specific class/interface in wouso. This mostly means separating it from the quite vague and a bit cumbersome Artifact class. Currently the back-end is enabled and the cpanel configuration. All current tests pass.

The two next steps are:
* completing the cpanel interface
* making sure the game interface works properly
* add setup script to insert achievements in the database during startup

Until we are able to properly manage the creation of custom in-game achievements, my proposal is for achievements to be statically defined (name, title, descripion and image). The only thing one would do in the control panel interface is enable or disable a given achievement.