Barkrex the kernel master

This latest update brings BARKREX into the game. This boss is a master of dogs and is incredibly dangerous! He will unleash his hell hounds upon you, and won't stop until he has been slain!

- Every wave ending with a 5 (5, 15, 25 etc) there will be a hell hound wave
- Every hell hound wave Barkrex has a 50% chance to spawn
- Multiple bug fixes

Have fun!