v0.0.0.78 - Tractor Beam, Salvage, Mining Laser additions

Have added in a few new items, to give another option for gathering ore. Can now add a tractor beam, salvage depot and mining laser. So passing asteroids can be grabbed, stored and mined. 

This is just the first iteration of these items. The tractor beam, will need a few changes.  As i currently only grab 'smaller' asteroids as larger ones have a habit of being pulled into your space station!  The player probably needs to be aware of this though so i need a way to show this. Player might also want to override this size limit.  Alternatively i could make the tractor beam much stronger, so it stops asteroids dead, and brings them in, in a straight line, but i dont think that looks ideal.  

May also want to select asteroids and tell lasers not to destroy certain ones, if they are heading towards your tractor beam to  be gathered. Currently lasers dont fire, once a tractor beam has 'grabbed' an asteroid, but will fire beforehand.

Few fixes with this release as well, mainly with turrets. Weren't showing if repair was scheduled, damage wasnt being cleared correctly and you could place a turret over another!

Nick