reverted the 1 shot 1 kill, the turret ai would not be able to survive higher levels

- turret won't waste tons of ammo in AI mode any more, but tries to do 1 shot 1 kill
- boss now has a health bar on top
- boss now throws bullets in random direction
- invader armour increases each level instead of each wave

- Invaders will now cycle through all 14 invader types, then show a Boss, then rinse and repeat for 10 times until the game is over.
- The turret now even has a few more braincells to deal with all the invaders in a more efficient way when in AI mode
- Changed explosion sprite a bit and also the locations where it spawns
- Made turret sprite a bit bigger so that a bullet can actually fit inside that thing before it's fired
- Turret will head for the invader which is nearest to ground to finish him off first, and shoot anything that comes across its path while heading towards that invader. (it used to just slide from left to right until screen edge was reached)

- fixed sprite misalginment with invaders boss