Implement Item deterioration

As an additional RP feature items should deteriorate during their use. Deterioration should make items less effective. To reduce the degree of deterioration an item has to be repaired.

Open questions:
* good formula for determining a negative effect on items based on degree of deterioration
* How exactly should items be affected by deterioration?
** Suggestion: Weapons atk should be reduced, armors etc should have less def
** At max deterioration an item should break?
* price ranges for repairing?