Handle CreatureTemplate like items

[fabio-t]

To properly implement the Character Creation (#20) I must handle the creature templates just like items and terrains. This implies a bit of refactoring into BootstrapSystem, or the creation of a CreatureSystem that allows instantiating creatures of a specific "type" at runtime.

It will probably be better if we define all creatures in a single file.

It remains the question or where to put player specific configuration, then. Some things shouldn't be modifiable, eg primary attributes are chosen at creation, not via a file. However, stuff like Sight should be configurable, unless I make Sight another primary attribute (that consumes attribute points at creation).