Refactored code to uniformly treat RPG systems. Added utils to list and load systems. Added base class to unify stat generators. Added module for S&W.

I refactored the code to allow uniformly adding, listing and loading RPG systems.

Here's an example:

```python
>>> import systems as s
>>> s.list_systems()
['dnd35', 'sw']
>>> sw = s.load_system('sw')
>>> sw.gen_stats()
{'Consitution': 9, 'Dexterity': 10, 'Strength': 9, 'Intelligence': 8, 'Wisdom': 13, 'Charisma': 4}
```