More weapons, turrets, new enemy AI

- added lasers
- added missiles (as a secondary weapon, right click to use)
- added turrets (placed by enemies, they will shoot you!)
- added a new enemy that spawns turrets
- added ability to shoot down mines
- changed kamikaze ship behaviour
- improved upgrade screen (instead of getting a damage increase you are given new weapon types)
- removed ability to create or join a multiplayer game (soon I will completely redo the network code, it's kind of a mess right now!)