Upgrade update!

This is probably the largest update to date. We finally have a more robust ship upgrade system that requires you to have resources to upgrade. There is also updated UI (still placeholder though), and a veeeeery simple enemy type that you might encounter if the asteroids don't get to them first...

Also, asteroids don't float into infinity anymore, but rather bounce back when they reach the end of the map. So there's a lot of more asteroids to mine now. They also don't get destroyed by the mothership's shield, but (mostly) changes direction, leaving even more asteroids to mine.