Nightmare mode + some new weapons.

New weapon: Shredder. It launches lots of bouncy discs. New weapon: Hyperdisc. It launches a really big, laser guided bouncy disc. (Both are dropped by the dreadnought on Electron Storm) Added a nightmare difficulty which doubles all enemy spawns (including bosses). It has its own scoreboards. Gyrojet howitzer and flak cannon now have better detonation control when aiming with a mouse. Scattergun shoots more projectiles but has a bit more scatter. Tweaked some enemy spawns, stats and AI. Dreadnought now has a particle effect on its engines. Some UI fixes.