Implemented a statistics system. I believe this feature was originally requested...

Implemented a statistics system. I believe this feature was originally requested by Ryan and the aim is to provide more replay value.
Currently, it just keeps track of score. In future, it could keep track of other things like: min number of jumps, max number of enemies killed, min number of shots, min time needed, etc.
When a better value is reached after playing the level again, it is replaced in the old statistics.
Worldmap is the one in charge for saving statistics.

TODO: draw current score and other stats of the current level in world map. I am thinking in drawing it in the Jump'n Bump way, that is using fade on text.

I had to use LispWriter when saving a slot.

SVN-Revision: 1911