Added the new stretch goal bosses from the Kickstarter!

Based on playtester feedback, we've made some awesome new changes:
- Added a level-up mechanic. Now players have to slay monsters in order to unlock their character's ability.
- Added a new damage type, the black Wound. This symbol indicates that ALL heroes in the fight take a damage when the monster wins combat.

Added 2 new bosses and included new art for several snacks.