v0.3.4: Rail Gun

Rail Gun is now released

1. New weapon just dropped: the Rail Gun is a single-action(?) rifle that fires one high-powered bullet per second. The bullet pierces all enemies it hits.
   1. The base projectile damage is 80. I think this is maybe too high.
   2. The base fire rate is 1 per second.
   3. You cannot click and hold the Fire button. You must click every time you want to shoot.
2. I finally got around to properly ordering the game's objects' z-indices. This means mostly nothing, but it means the ship weapon is now correctly rendered beneath the main ship, not on top of it.
3. I will revisit the Firing Drone soon. As I add more weapons, I am finding myself reconsidering the initial implementation of "this drone fires one of your weapon's bullets" because the behavior changes so much from weapon to weapon. For example, if you're playing Engineer right now, there's no reason for you to pick anything other than Rail Gun. Even outside of the Engineer class, Firing Drones that are shooting Rail Gun bullets are doing 16x the damage dealt by Heat Ray bullets. My initial thought is to always have the Drone shoot a basic Auto Cannon bullet, but I need to sketch it out.

Going forward (maybe later today) I will change up the in-round UI slightly to display (A) a new kill counter in the top-right; and (B) change the gold counter to display the current run gold, not the current round gold. Knowing how much gold you made in the given round is not that helpful, but knowing your total available spend is.