Now it's a game (v0.3)

Alright, now this looks like an actual game, even if still pretty rough. I've made sure to finish up the most basic aspects to turn BfE into a game with a fail condition to better test the overall experience.

Change List:

• New Start and Game Over screens;
• An actual fail condition with a working HP count
• Also, a score count is in place and working;
  
• Instead of disappearing, enemies now attack the tower;
  
• Now only one enemy is ever hit by a single arrow, instead of everyone getting hit at once when enemies are piled up;
• Enemies are now smaller to make them a little more in line with the intended proportions;
• The ground's hitbox has been adjusted so that enemies aren't hovering slightly  above it;
• A few parameters have been rebalanced and updated on the GDD.