-changed Level class to save tiles in a big 1 dimensional array instead of

-changed Level class to save tiles in a big 1 dimensional array instead of
 vector<vector>. You can access a tile like this now tiles[y * width + x] is the tile at place x,y
-introduced new Serializable interface that is implemented by all objects that
 are able to save themselfes to disk
-added new lispwriter class
-changed level laoding/save code to use new serializable/lispwriter stuff

SVN-Revision: 1282