Add base powerup class
Add dash powerup class
Make Powerup inherit from GameObject, not Physical
Close path in Powerup render event
Add DashPowerup to level
Revert "Make Powerup inherit from GameObject..."

This reverts commit 73b0fc1079d0667bdc69f2572b92604e873f374e
Set powerup gravity to 0
Rename Powerup's collision handler method
Implement initial powerup collision code
Rename DashPowerup's collision handler method

I'm an idiot
Fix overlap detection code
Fix wrong variable reference
Merge branch 'topic/ghi/33' into dev

Fixes Github issue #33