Basic "gold" (currency) mechanics

Added "gold", which can randomly be emitted by ships when destroyed.

You can collect it, but it doesn't do anything yet. Obvious next step is to add "shops" of some kind... the easiest way is as special nodes of the map, which when visited, open a shop menu. (As opposed to being an area you can actually fly around in with your ship.)

There are also new types of ship which can "mine" asteroids for gold, although these ships aren't placed anywhere in the game yet.