added Category model for #21
added Category model for #21

Fisiu/calendar-oswiecim,Fisiu/calendar-oswiecim,firemark/calendar-oswiecim,hackerspace-silesia/calendar-oswiecim,hackerspace-silesia/calendar-oswiecim,hackerspace-silesia/calendar-oswiecim,firemark/calendar-oswiecim,firemark/calendar-oswiecim,Fisiu/calendar-oswiecim