Create a world description table

[corot]

With it we can implement issue #17 while conserving uuids. Also will allow to store other informative fields as a world description, world geometry (2D, 3D, limits, etc.), multi-stage worlds, etc.

Also relates with issue #9: database export files will be organized by worlds, according to this issue.

[corot]

I have created the table but I'm not making any use of it. In fact, it's not clear at all its utility, as I have completely dropped world_id, and its main goal was relate world name and id.