Build 0.4.3

A few notable changes:
- Added boss enemies. These appear before every 20th stage, prior to the warpstone stage. These enemies are meant to be a lot stronger than other enemies.

- Dealing and receiving damage now results in a small text pop up to display the damage amount.

- Added gold. You receive gold from killing enemies. The amount gained is influenced by the stage number as well as the user's Luck stat.