Update: Mothership

Added a bunch of new features and fixes:

New: Mothership
At level 4 and higher, players in a single player or co-op game will have to face down a huge mothership. The vessel comes with 8x turrets with a variety of weapons and launch fighters that track down and shoot players.

New: Brood Ship
From level 3 onwards, players will be hunted down by sinister brood ships who. These ships slowly pursue players, releasing spawns that tirelessly follow follow players.

- Small UI improvements
- Gameplay balance 
- Minor performance improvements